The following rules allow Skill rolls, Attack rolls, and Characteristic and Perception rolls to be made on percentile dice (d100) rather than on 3d6. This has two advantages: first, it eliminates the 'breakpoint' values of Characteristics for these rolls. A Characteristic of 13 will no longer be a 'magic number' -- Characteristics of 12 or 14 will be equally cost effective. Second, most people find it easier to understand probabilities expressed as percentages than as chances of rolling under a target number on 3d6; people have a better intuition for "74%" than for "12-" even though the two are almost identical.
Each skill that had a 3d6 roll under standard HERO rules has a percentage score in this new system, e.g. Demolition 68% rather than Demolition 11-. The percentage scores are calculated as follows:
The GM may have some background skills use a Characteristic other than Intelligence, or may have some skills use two Characteristics, e.g. PS: Surgery based on (INT + DEX + 40%) for 3 points.
When using a skill, characters roll d100 against their Skill percentages. A successful roll indicates successful use of the skill. Skill Levels add +10% to effective skill for each level used. In general, each +1 or -1 modifier under the 3d6 system gives +10% or -10% under the percentile system.
Characters with a modified skill of 100% or more should be granted automatic success when performing common, non-stressful tasks. Characters with a base skill of 60% or better should be considered "competent" or "expert." With the +40% bonus for routine tasks, they have a modified skill of 100% -- don't bother rolling the dice. But under unusual and exciting conditions (such as are often found while adventuring), the GM may wish to allow a small chance of failure even if the modified skill is very high. He does this by having the character "confirm" successes when has a 100%+ skill and rolls '00' on the dice. To confirm a success, re-roll the dice against half the skill. If this second roll is a success, the character succeeded with the skill. If the second roll failed, so did the character.
Example: A character with 106% in Combat Driving makes a Combat Driving roll and gets a 00. He must re-roll the dice and get a 53 or less in order to succeed.
On the other hand, a failed roll, whether a failed confirmation or an "ordinary" failure, should not usually indicate total, catastrophic failure. If the character has time to keep working on the situation, the GM should allow repeated rolls with a cumulative -10% modifier on each roll. On the third hand, if the character fetches tools, takes extra time, or otherwise acts to give himself a bonus, then the GM should allow the roll and not apply the -10% modifier.
When two characters use skills in direct conflict with each other, each character should simply roll against his skill percentage. If only one character is successful, then that character (obviously) wins the contest. If both are successful (or if both fail), then the one who rolled highest on the dice wins the contest.
Special case: If a character has a skill above 100% it "wraps around", and low rolls count as very high successes. If a character has 128% in a skill, for example, then rolls of 01-28 should be read as "101-128" and a roll of 29 will be the lowest roll possible.
When one character uses a skill to set up a situation, and the other character must overcome the situation by using a skill, the first character starts by rolling against his skill. If the first roll fails, the situation wasn't properly set up, and the second character need not roll at all. If the first roll succeeds, the second character must roll as well. This second roll has a penalty based on the "10's" digit of the first roll: -0% if the 10's digit was "0" (i.e. a roll of 01-09), -10% if it was "1", -20% if it was a "2", and so on - as long as the first roll was a success. Remember to apply "wrap-around" for skills above 100%. Wrap-around can raise the "10's" digit to 10 or higher, and thus can push the penalty to -100% or more.
Example: Andarra has Computer Programming at 60%. She tries to set up a security block on her computer system, taking extra time so as to get a +10% bonus. She thus needs to roll a 70 or less to successfully set up the block. She rolls a 36, setting up the block and giving any intruders a -30% penalty on their Computer Programming roll to hack into her system. If she had rolled 52, the penalty would have been -50% to any hackers, and if she had rolled 74, she would have failed to set up any block.
Later, Andarra has her friend Pfred set up a better block. With his levels and special equipment, Pfred's modified Computer Programming skill for this task is 114%. Pfred rolls a 08. Normally this would give a simple block with no penalty to hack through, but Pfred's skill is high enough that wrap-around comes into play and the 08 is read as "108." As a result, hackers attempting to attack this block have a -100% penalty to their Computer Programming skill.
The "meta-rule" here is that the higher the roll the better one has performed - as long as the roll doesn't go over one's skill percent.
When using a Complementary skill, roll d100 against skill percent as normal. If the roll is a success, take half the 10's digit (round up) and apply it as a bonus to the main skill roll. Thus if a "54" is rolled (and is a success), then the main skill gets a 30% bonus. (Half of 5, rounded up, is 3 - which translates to a 30% bonus.)
Remember to apply "wrap-around" for skills above 100%: A roll of 23 against a skill of 130% is really "123" and gives a bonus of 60% to the main skill.
Note that the minimum bonus given by a successful complementary skill roll is 10%: A roll of 01-09 (without wrap-around) will give a bonus of 10% rather than 0%.
As noted above, each +1 or -1 worth of "standard" modifier gives a +10% or -10% modifier in the percentile system. For example, the Skill Modifier table on page 18 (BBB or CD) would translate as follows:
| Modifier | Circumstance |
|---|---|
| +30% to +50% | Routine |
| +10% to +30% | Easy |
| -10% to -30% | Difficult |
| -30% to -50% | Extremely Difficult |
| -50% or more | Sheer Folly |
| +10%/level | Extra Time (one or more levels down on the time chart) |
| +10% to +30% | Character has extensive knowledge of the object of his Skill Roll |
| +10% to +30% | Character roleplays the Skill use well |
| +10% to +30% | Using good equipment in connection with the Skill Roll |
| +10% to +30% | Excellent conditions for performing the Skill |
| -10% to -50% | Poor conditions for performing the Skill |
| -10% to -50% | Extremely strange or weird (to the character) object to perform skill on |
| -10% to -50% | Lack of proper equipment (if appropriate) |
| -10% to -30% | Combat conditions, for Skills not normally used in combat |
When using percentile dice, a Characteristic roll is [(2 x Characteristic) + 40%]. An Intelligence or Perception roll, for example, would be [(2 x INT) + 40%]. As is the case with skills, each +1 or -1 modifier in the standard HERO rules would translate into plus or minus 10% on the percentage Characteristic roll.
Percentage Combat Values are calculated as follows:
When making an Attack Roll using d100, a successful hit requires the character to roll over the target's DCV but less than or equal to the character's own OCV. Combat Levels and other modifiers apply as usual: Each +1 or -1 modifier in the standard HERO rules gives a 10% modifier in the percentile system.
Example: George (OCV 96%) attacks Agent Fred (DCV 30%). George's d100 Attack Roll must be greater than 30 and less than or equal to 96 for the attack to hit.
If a character has an OCV greater than 100%, it "wraps around" (and effectively eats into the target's DCV from the bottom).
Example: Speedy Sam (OCV 129%) attacks George (DCV 36%). For Sam, rolls of 01-29 count as "101-129." Since rolls of 101-129 are above George's DCV and less than or equal to Sam's OCV, they hit.
If both OCV and DCV are above 100%, simply drop the 100's digit from both, i.e. OCV 160% vs DCV 132% simplifies to OCV 60% vs. DCV 32%.
When making an autofire attack, each full 20 points by which the d100 roll exceeds the target's DCV indicates an additional hit on the target. For OCV 87% vs DCV 30%, a roll of 31+ indicates one hit, 50+ indicates 2 hits, 70+ indicates 3 hits -- and 90+ indicates a clear miss. If the attacker's OCV "wraps around", then the autofire count does too: For OCV 115% vs. DCV 30%, a 31+ equals 1 hit, 50+ equals 2 hits, 70+ indicates 3 hits, 90+ or 01-09 (read as 101-109) indicates 4 hits, and a roll of 10-15 (read as 110-115) indicates 5 hits. A roll of 16 (too large to wrap-around) indicates a clear miss.