Good question. Here's how I do it. Scope Pre cant (20 MOA) is invariant to the measurement. The gun is my latest toy from Kirby. It's a mdl 700 .308 with a Holland QD brake, trigger job and scope rail holes bored out to #8. I added the Loopie 3.5-10x50 VX-III and the stoney point target nobs. Not shown in the pic is the muley pod. Last weekend it was shooting bugholes from the Harris bi-pod with my non-fire-formed brass doing load development. Everyone should own a .308
Because of a bug in the forums software, I had to create the page on HERE
on my site. The experts will likely have a more accurate approach and a faster approach - but this is fast, easy and accurate enough that any error is pure noise. (Note the rough guesstimate error is pure noise).