I use a dice pool, which trades away precise percentage markers in favor of ease of use, and a few other features like mixing together several checks into one roll or adding extra effort to an action. However, the theoretical maximum number of successes this dice pool can generate is 8.

* Basic actions require 1 success.

* Normal actions require 2 successes (this is the most common kind of action).

* Difficult actions can require between 3 and 5 successes.

* Nearly impossible actions can require 6-8 successes.

* Truly impossible actions require more than 8 successes. You can't possibly roll that, so it's automatically impossible.