Greg,
If it is for 12 ga., use the ring(.700) and slip it over the ball bearings while trying to keep it even on them. Set the dial to zero. Insert into muzzle as far as you can go, usually about 11 1/2". Read dial now, if 12 ga. should be +.025-.032. That would be .700 + .025-.032 or .725-.732 for bore size. If doing this on a double barrel do the same for the other side. Then insert gage and set dial to zero and read choke size coming out. Readings will be minus (-). Example coming out dial reads -.039, so .039 from bore size of .729 = .690 or full choke.
