Let me start by saying I attended the same clinic, the one I attended was in Akron a few weeks ago. Both Mo and Del are extremely knowledgeable and friendly.
Now, on to your shorter span. Just because they suggest you use a shorter span does not mean that the shorter span is the way for you to go. It is simply a suggestion. If you hurt your ring finger (knowing what I know about Mo suggesting drills) it is probably because he put too much forward pitch in your ring finger to add the axis tilt. That would somewhat stretch your ring finger a bit.
Mo suggested shortening my buddy's span from 4 3/4 (I had already shortened it to 4 3/4 from 5inches), to 4 7/16. No way could he throw the ball, he just kept dropping it. He has since junked that idea. I on the other hand loved the shorter span Mo suggested for me 2 months ago.
NOW----I think Mo is a genius in the tech and drilling fields of bowling, but let me say this: I had him measure a different friend's span 2 times in one day, and got 2 totally different suggestions. So take that for what it is worth.
So, if I am correct and Mo suggested a large amount of forward pitch in your ring finger, just try to plug the finger and move it to a little less forward pitch.
goodluck
