The true reading is exactly at the muzzle. This tool is only "advisory." Constriction must be measured at the start of the choke taper and at the minimum diameter; this should be at the muzzle or a bit back if the choke is cut with a parallel that has gotten cut divergent. Don't worry about exacting measurements. Shotguns are sufficiently variable pattern to pattern (shot to shot) that choke is an average. It takes several thou difference in constriction to really make much difference in practical performance.