If you read Ron Clifton's articles, he talks about 2 things for the thumb pitch. First, a forward pitch should be employed when a bowler's wrist is NOT straight or NOT cupped. If a bowler's wrist is weak or lazy (his terms) the ball weight is transferred to the thumb and the ball feels like it will fall off which encourages thumb gripping.
"If you find yourself in a situation where the ball wants to fall off of your thumb no matter how tight you make the thumbhole, then you need to take a look at your thumb pitch. You will most likely need to move the pitch more forward."
Second, if the bowler cups the ball through the swing, he keeps the ball weight on his fingers which is recommended. He even recommends a
wrist support to accomplish this. In talking with a couple of ball drillers, the ball will hang up on the thumb with forward pitch and a cupped hand, so reverse is suggested.
"If your wrist is weak or lazy, then it will be open just before the bottom of the swing. This means that your wrist is not straight or cupped, but bent back. This transfers more of the ball’s weight to the thumb so the ball falls off. Try to keep your wrist cupped and carry the ball’s weight on the fingers."
Therefore, it seems that the correlation of how much forward or reverse pitch should be employed is not related to the span as it is directly related to the amount of cupping or weakness a bowler's wrist has.
Never saw anything with this analysis.