you can also use a drill press, chuck the assembled upper upside down in a machinist's vise, set a level across the flats on the bottom of the upper and level the assembly. Drop the drill until it just kisses the top of the FF then walk the handguard across the tip of the bit, the bit will score the highest point. Center drill the score and install the stud using a hex nut, don't drill and tap the handguard, it's a very weak interface.

Quoted: you can also use a drill press, chuck the assembled upper upside down in a machinist's vise, set a level across the flats on the bottom of the upper and level the assembly. Drop the drill until it just kisses the top of the FF then walk the handguard across the tip of the bit, the bit will score the highest point. Center drill the score and install the stud using a hex nut, don't drill and tap the handguard, it's a very weak interface.

Tweak-

I do this so often that I got myself a reverse spotfacer.
It's a bitch to get into place, but it sure does a nice job.
Then I set the nut in the spotface with a good drop of blue loctite.
No way that stud is coming out.

One other thing to mention if you are doing this for mounting a Harris bipod, you will also want to add a setscrew through the handguard tube and into the barrel nut if your handguard tube is a thread-together 2 piece.
This will keep the handguard tube from coming loose when you are wrenching your rifle around against the bipod legs on the ground.
It happened to me once in a match many years ago and I have added this to every aluminum handguard tube I have put a stud on since as a precaution.

Quoted: I do this so often that I got myself a reverse spotfacer.

good point about the addition of the alinement screw to the handguard, ran into that problem several times. I built a jig that alines on the pin lugs of the upper, just set it on the lugs and run a transfer punch through the hole. Still have to level the upper on the press but the hole is started TDC.