Wide vs. narrow stance are VARIATIONS, not errors.
As long as you're not stressing your shoulders, elbows or wrists, and as long as you're keeping your core engaged so you've got a nice straight line from your head all the way down to your fulcrum (whether that's toes, knees or somewhere else on a stability ball), do whatever variation you like.
I'm doing mine with a fairly wide stance, personally. I just measured about 21" between my thumbs - which I keep all my fingers somewhat splayed to distribute the load - vs. the 15" (38 cm) between the bony protrusions of my shoulders when I was measured for handlebars.
But maybe that can be another challenge - once we've done 100, go back and do it again with a different stance. 
yeah right
Speed comes from what you put behind you. - Judi Ketteler