Bill Taylor does a nice job describing the feel of a proper span on the fingers in his classic book,Fitting and Drilling a bowling ball. (Many will say the book is very old OK)
Page 16-18 discusses the issues above very clearly and completely!
Bill points out that the standard of 3/8 of an inch over the center of the hole is PERFECT for conventional grips because of the flexibility of the 2nd joint is right near 90 degrees.
He goes on to say on page 17 that in a fingertip the first joint near the finger tip will not bend at 90 degrees and since the TWO joints are taking up the 90 degrees of bend together thus forming a SLIGHT ARCH. This arching slightly reduces the span in relation to the 1st joint near the fingertip.
This is why he mentions that the 3/8 inch last joint over the lip of the hole which is so perfect for conventional grips does not apply to the fingertip. He much like NotClay below mentions a starting point of the midway point of the last two joints being just over the lip.
Going on he states on page 18 of this great book is that the FINAL test is that where the bowler feels the pressure on the first joint during the swing! He states,
"As the bowler swings the ball, with minimum hand squeeze, the pressure should be on the first third of the fingertip section NEAR the joint!". Here he shows a picture of pressure or weight of the ball closest to the joint closest to the fingertip.
On page 19 he goes on to state, "Then as the thumb departs, the pad of the fingetip will exext consistent lift". At this point he shows a picture with the complete area of the tip of both fingers feeling uniform pressure.
Finally on page 19 he shows pressure on just the tips of the fingers. He describes as during the swing the ball resting on the final pad of the fingers completely and then during release shifting just to the final 1/3 of the tips as being a slightly too narrow span.
Final thought from me..
Bill admits that the 1/2 point is just a starting point. Bowlers such as NotSoHotShot and me ofter find because of very flexible joints near the fingertip(I've got them too) that our fingers can take up a little more span than this starting point.
Bill states that by following this procedure most bowlers end up within 1/16 of their ideal span. He then goes on to state AGAIN on page 28 that the final test is the pressure during the swing BEFORE the thumb comes out is NOT on the full pad of the last pad of the fingers but closer to the joint. THEN the last feel is of the ball AFTER the thumb comes out, being on the full pad! NOT just the last part of the fingertip. (This for me is the most important thought for my own grip).
I hope this has been helpful. Note many bowlers today seem to go a little shorter than this. However I note that to my humble eyes Pete Weber seems to take every bit of a full span and last I heard some people have thought his release looked pretty good!
Regards,
Luckylefty
PS in describing taking the span measurement Bill Taylor states on page 22
"Insert the thumb, and while the bowler keeps his thumbface in contact with the wall of the hole, and center finger fully stretched, note where the span mark falls on your measuring device. Do this several times until a measurement repeats. Do the same with the ring finger.
Just like NotClay above he describes a span mark for the fingertip as dead center of the last two joints. Differing with him he notes with the center finger fully stretched.