While I do not think that this is necessary for a properly machined lower, I have had folks ask me about them so I figured I'd post this.
I set the mill up like this, using the following datum points. The normal "Y" axis zero, the flat area just behind the PG mounting boss for the "Z" zero and .156 to the right of it as the "X" zero.
Using a 5/16" end mill I machined along the X axis .265 to the left into PG mounting boss. You do not need to mess with either the Y or Z just leave them locked.
Drilled it out with a #3 bit.
Tapped it to 1/4"-28
Added 1/4"-28 X 3/4" set screw. It sits well below flush when fully installed
I set the mill up like this, using the following datum points. The normal "Y" axis zero, the flat area just behind the PG mounting boss for the "Z" zero and .156 to the right of it as the "X" zero.
Using a 5/16" end mill I machined along the X axis .265 to the left into PG mounting boss. You do not need to mess with either the Y or Z just leave them locked.
Drilled it out with a #3 bit.
Tapped it to 1/4"-28
Added 1/4"-28 X 3/4" set screw. It sits well below flush when fully installed