So the base mechanic is similar to that of 7th Sea/L5R. There are specific modifiers to the TN based upon the effect that is desired, such as range/area/duration/length/etc./or GM modifiers.
This said would I be better to use an XP advancement system? Or would a system that requires players to fail and succeed a certain number of times to advance a skill? I really like the Pass/Fail advancement system, and others similar to it, but I feel as though this may be a problem if the players have a really good idea as to the TN they will face.