I project a line perpendicular to the center line. then I double check my measurements. If you have a metal span ruler like Raider mentioned, you can hold the zero on the thumb hole mark and swing it out and see where the span on the metal ruler and where the perpendicular line meet, this should be your true span. There is only the Jayhawk precision grip tool that takes the arc into account in one tool. More over, to truely hit your span, you usually end up taking out half of the perpendicular line out with your drill bit...or if you scribe the lines..which is a finer point then you can get with a grease pencil..you can hit the span that way.
You just need to try laying balls out and figure out what works best for you.