My test is, with the barrel and tang inlet all the way. The barrel should fall out if you hold the gun upside down. Lay it on the bench and pick the stock up, the barrel should stay on the bench.
After the barrel is inlet I file a slight draft on the lower 1/2 of the side flats. It don't take much and I don't file under the lock. I also file the sharp edge off the bottom of those flats so after finial finishing is done I won't shave wood off the channel when the barrel goes in.
I want the barrel to seat on the bottom 3 flats and just touch the sides or at least be real close.
Right or wrong, that's how I do it.
Bruce