I continue posting in ages old threads, but pardon me, I'm new to this forum.
I think that the system that reduces fumble ranges, also needs a system that increases fumbles ranges somehow.
We've dropped original fatigue system and created a 5-step fatigue system, in which every level lowers the skills and increases fumble range by at least one. For example,
Winded character suffers -10% to all his skills and gets +1 to all fumble ranges.
So, a fresh fighter could have minimum of 1-2 fumble range at the beginning of the fight, but as the conflict gets prolonged, he tires and starts making more and more mistakes or even fumbles.
I think that this increased fumble range could be applied to original fatigue system as well, given some kind of intervals for additional fumble steps.