Many of the top builders use the "Three times tight" method.
First they coat the nut threads with a NON-graphite anti-seize compound.
NEVER use graphite around an aluminum part, graphite damages aluminum.
With the threads coated, tighten the nut to just over the minimum to mid range torque, then back it off and tighten again, back it off and tighten the last time.
This tighten-loosen process force mates the screw threads.
Once you've tightened it the last time, check where the notches for the gas tube are.
Most builders just muscle the nut to align with the gas tube instead of backing it off.
If it's too far, they continue loosening and tightening the nut.
Each time the nut will screw on a little farther until the groove does align.
Backing the nut off any real amount to align risks vibration of the barrel ruining accuracy.
There are any number of ways to fit a barrel but this is one many top builders use.