I use a telescoping gage and a micrometer to measure the distance between the races with the casers bolted together, then take a measurement of the thrust washer surfaces on the flywheels with an old non calibrated type caliper. I use a dial caliper to measure between the points of the old style caliper. Subtract the flywheel measurement from the race measurement, divide by 2, add clearance and you should be in the ballpark. I assume you are trying to order a minimal number of the thrust washers for the job. The problem is, if you are going to center the flywheels in the cases, you may need to use completely differant size thrust washers.