Install alignment of FSB

[HKGuns]

I am building up an A2 upper and it is full of "firsts" for me.

I have the upper assembled and a 20" Colt barrel waiting on a nut and delta ring.

I installed the A2 rear sight, which was a bit of a challenge. I had to make my own slotted wood dowel to get the spring compressed on the bottom, so I could get the cross pin through. Went pretty smoothly after I figured out how to make my own tool.

I don't want to drill on the barrel, so I got a JP clamp on FSB and am trying to figure out the best way to ensure I don't cant it on myself.

What methods have you used to ensure you don't cant your FSB when installing? Any and all suggestions appreciated.