Basically it is this; you take your I.Q. + M.E. + (0.5 * P.P.) + Your
I.Q. Bonus + 1d6. If your P.P. is an odd number, round down. At first level
you add a one time bonus of 1d6. This is because even two people with the
same "attributes" would see things differently. Also at each additional
level after one, the person would gain a +2% to their perception.
Example: You are level 3 and have an IQ of 21, ME of 15, a PP of 19, an I.Q. bonus of +7%, and you rolled a 4 on 1d6 at first level. Your perception would be 60%.
This will be a percentile roll, that can be modified do to some skills that would help developed a persons perception. Skills like detect concealment, and detect ambush; each of these skills adds a +5% bonus to the roll. Advanced powers, like Heightened Sense of Hearing, SV: Advanced Sight (when active), and Radar, will give you a +10%. Heightened Sense of Smell will give you a +5%. These bonuses only apply when that power might be helpful to notice the event. Night Stalking power would give you the skill bonus, but only when the power would become activated. Robotics and Cybernetic implants like optics, audio, and motion detectors also get bonuses. If it is like a given power of some kind, (like Advance Hearing is like Heightened Sense of Hearing, and Radar is like Radar) give it the same bonus. GMs always have the final call as to weather the power could be used to help the person in this situation.
Example: Johnny drops an empty clip onto the ground, he is hiding on the other side of a wooden fence, Kimmy has Heightened Sense of Hearing and gets an extra +10% bonus to hear the clip fall as she moves toward the fence. She now has a 55% rather then her normal 45%.
Basically the idea came from the fact that there are a lot of games out there that give you a chance to notice things just because, and I felt the game needed one. It is based on the fact that you would be able to see things better depending on how intelligent you are, and how fast you could react to something. A person with a high intelligence and a strong mental will would be able to notice something that was out of place better then someone who isn't. It's the GM's job to determine if and when the perception skill should be rolled.
Here is a short list of some bonuses and penalties that one could suffer:
Very Easy: +20%
Normal: Strait Roll
Very Difficult: -40%
A roll of a one (01) on percentile dice always makes it, given that
there was an actual chance that the person could be spotted. When dealing
with prowl, you use the rules like this.
If the person makes there prowl roll, then it is difficult to notice them if you are facing in their direction, but a very difficult if you are not. If the person fails their prowl roll it is a easy roll if you are facing their direction, but a moderate to see them, if you are not. That is because the person is actively trying to be sneaky, and hiding in shadows, walking very slowly, that kind of thing. Actively searching for someone or looking for something you know is there gives you a +10% to your perception rolls. But this will also take extra time.
To determine what the penalties or bonuses might be consider the background noise, possibility of heavy cover, cars moving pass when trying to see someone on the other side, and even the weather. All of these play a part in how well you might be able to see something. Itís all up to the GM on how to judge the roll.
If a person rolls a double zero (00) this is considered a critical failure. Take that person's perception; subtract it from 100 and then divide that amount by 10. (100-PER)/10=X. This is a penalty that the people around him suffer because he has just done something to draw attention to himself, and distract the others from there search. Also this gives a bonus to anyone looking for that person, or the people with him.
Over all I think it is fairly simple, and easy to use. A good rule of thumb is to have the group make several perception rolls when they actually don't need them, and maybe keep those numbers in mind when they are needed. Doing this will trick your players, so they don't go, "Oh I have to make a perception roll, I guess there is someone out there or something important I have to see. Darn I failed, maybe I should stop and look again." Players will use whatever they can to get an edge, I don't think they should use there GM.