There are plenty websites that show how to properly measure a shotgun barrel. Their are zillions of 18 in barrels out there.good job i would say - if i was cutting one i would cut it to 20” just to be safe. You arent going to notice that much difference and you know it is legal. (That is until they decide to change the rules)
I cut mine pretty close to 18 3/4 due to the vent rib. I know it’s legal.