I used the one on wildsnow (https://www.wildsnow.com/12587/fritschi-vipec-mount-template-jig/), worked well. doesn't have the BSL lines like here but it works.

I did this method on my last mount. It worked great. I made marks closer to the tips and tails to minimize any errors. Then I connected with my 48" straight edge. No measuring, no arcs. Done and done.