I believe it's just a standard reference point when measuring and drilling the finger holes.

From my experience with a few guys who drilled on the PBA truck,  they like to drill the thumb hole 1st and then drill the fingers.   They measure out the lines ahead of time and use the 5/16th or some other reference intersecting point for the finger holes.

For instance when I drill my own balls, I like to use 1/2 of the finger size from the bridge line.  This would put the span line right in the center of the finger hole. 

Some guys do it different.  Not sure where the 5/16ths came about, but somebody else maybe able to explain that better.

The CLT would be the 1/8Left offset and the 5/16th measured off the center line?   I guess the 1/8th L and the combination of spanning the fingers at 5/16th from the center line makes the CLT.


Maybe it's just a Norm Duke thing.   The article is a bit miss leading as it implies that the PBA drilling truck uses the 5/16th reference point on all of the balls they drill.

It must be a Duke thing, because non of the other articles in the series mention a 5/16th measurement.   

It just creates a consistent span measurement regardless of bridge width.  Nothing more.  The picture illustrates that very well.  Notice the span lines aren't touching the actual finger hole edges, but instead the horizontal tangent line to the finger hole edges 5/16th from the center line.