I actually modify the fumble chance by the Difficulty Rating of a maneuver.
Routine 1%, Easy 2%, Light 3%, Medium 4%, Hard 5%, Very Hard 6%, Extremely Hard 7%, Sheer Folly 8%, Absurd 10%
But I also allow a "Recovery Roll" by the fumbler to reduce the "bad side effects" of a fumble.
Then again, one of the reasons for Fate Points is the inopportune fumble...