I use 6" inside leg calipers gradualy turning them down until I can juuust barely feel some drag, then measure the span with a micrometer. It will be within .001 or .0015; close enough for me. Had to grind down the sides of the legs so they would fit in a 20 bore.
There have been some guns that still didn't throw the size patterns they should have according to bore and muzzle measurements. Probably because the way the choke is tapered in a particular gun.

Don't sacrifice the future on the altar of today