I like the Jonell span gauge, but if you go that route make sure to get the one with the longer extension on it.
I find this gauge allows you to effectively find the correct span regardless of bevel/ pitch, where the old styles don't. Just remember that if someone has a lot of bevel, even this gauge can give you a hard time finding the correct measurement. I know that if I try to span out my own equipment it can be tricky off of a drilled ball.
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-Chris: DJ's Pro Shop : Auburn, MA