CORE RPG RULES
Iconoclast -- RPG -- Modifiers
Any and all modifiers are applied to the target, never the roll. Everything you will add or subtract will affect the target. By modifying the target number, not the roll, we make it easy to see what the roll was and tell immediately if it succeeded or failed. There is no modification of a die roll. This makes life easier in the long run.
In general, modifiers are left up to the administrator(s) to apply on the fly. When in doubt, apply no modifiers at all. But if a situation obviously requires modification, then you can use the following as a rough estimate of what penalty to apply:
-80 Highly Difficult
+40 Very Easy
+100 Child's Play
Penalties or bonuses of more than 100 should be few and far between, since they severely change the flow of the game. If the situation absolutely allows for it, you can of course split the difference and apply bonuses or penalties of increments other than 20 points; however, keeping any additions or subtractions to increments of 20 allows the game to flow much more quickly in most situations.