I am with Acer on this. I mark the barrel for the touch-hole (vent) so that the threads of the vent liner (I like the White Lightning liners) just barely miss the face of the breech plug.
Then I build the whole gun around the vent, centering the pan on the vent location, then locating the lock plate, then locating the sear, then locating the trigger, then locating the butt-plate.
Best Regards,
JMC
John Cholin