Daniel,
I think that's pretty common for wood recurve risers, long bows maybe not so much as they can be narrower in the grip area. I've seen ways to measure it, but as has already occurred to you it's not as easy as it sounds. They're not like CNC metal milled risers with perfectly flat/perpendicular surfaces to reference off of. Maybe someone will chime in. The easiest way is to ask the bowyer...and hope he's right. I'm assuming you're shooting 5/16" diameter standard shafts. If you go skinny your results will vary and you may have to build out the sideplate.