the following graphs demonstrate most of the features of the system, such as what happens when you add Modifier Dice, cancel Modifier Dice, convert Add Dice to Modifier Dice, and change die types. In particular, note that canceling Modifier Dice (1 Bonus and 1 Penalty = no Modifier) is not the same as leaving them in. However, for all pools over 2 Add Dice, it is IMHO plenty close. As an option, if you're more concerned about statistical fidelity than simplicity, you could eliminate the cancelling rule. However, since about the only time it should come up is in characters working together, or when using a supporting ability with a negative attribute, it shouldn't be too much of a problem. also, not cancelling has a more normative effect, so if you prefer a more dramatic campaign, cancelling is the way to go--you'll get more extreme results.these first charts just show the raw probabilities for a lot of basic rolls:
Sorry, don't have most of these ready yet.