QUOTE (Seth @ Jan 27 2011, 09:36 PM)

Let me point that it does:
SUppose I need a threshold of 3 to see you.
Without the spell 3 successes with it
If you get X successes with the spell, I now need 3+x successes.
Basic algebra shows that this is exactly identical to subtracting x from the number of successes
Adding to the threshold is identical to subtracting successes (except for the rule of 1)
So this spell is about three times more effective that concealment or chameleon: they reduce the die you roll. This reduces the successes that you get: a ratio of about 3 to 1.
I see what you are tying to say, however the way i look at is the following;
For each point of threshold you need to roll 3 dice, by law of averages to be reasonable assured that a hit will come up. With perception (the skill this spell interacts with) Just by actively looking you get a +3 Dice pool bonus, there is one hit. An average human has intuition 3, Another hit. And a perception of 3; that's a total of 3 hits minimum by the laws of averages for an average cop, sec guard or Johnson. But begin to add to the threshold and you begin to make it nearly impossible very fast.
The average Pedestrian is a base threshold of 2 In my running example. The force 3 spell's 3 hits add 3 to the threshold, bring it to a total of 5. Now since it takes by the law of averages 3 dice to get one hit you now need a total of a 15 dice pool to effectively get it each time. That takes it from a Average cop, Corp Sec, and J to notice to the high end of a human; requiring 6 Intuition and 6 perception and actively looking for the final +3 bonus. Therefore a Force 3 casting makes it nigh impossible for the average person to take note; as without the skill the average person only rolls 5 dice(3 for Intuition, 3 for actively looking and -1 for defaulting on perception).
However If i change the spell to reduce the dice pool, -1 per hit for example; it would change our example like this: The base dice required to beat the threshold 2 test, with the modifiers of the force 3 casting with 3 hits, by law of averages is 6. For a average cop, Corp Sec or J; no problem. The +3 dice bonus from actively looking negating the spells modifier. However with their 3 Intuition and 3 Perception, they should still get the 2 hits by averages. While this still makes it nigh impossible for a average person as they would be rolling only 2 dice(3 for Intuition, 3 for actively looking and -1 for defaulting on perception as well as -3 for the spell's 3 hits). But as you begin to deal with observers that are in any way perceptive and or augmented you begin having to cast at much higher force ratings to be able to make it work effectively.
This difference in effect is at what forces it need be cast to be effective in a given situation, as well as balancing the drain accordingly. If it's powerful enough that a force 3 casting makes require higher skills and attributes or augmentations, it need a much higher drain than one that only needed two of those 3 at the same force.
I am trying to balance the spell as best as possible. So i am trying to figure out what the drain would look like with each version. The spell referenced earlier, Physical Camouflage, has a drain of (f/2)-1 and effects ranged combat as well as perception. While the spell i am going for only effects perception, and loses the effect if the subject draws attention to themselves.
The drain for the version that add to threshold would have to be higher, i just don't know how much, i would like suggestions for both versions.