The crook of the arm measurement is only a rough rule of thumb and more used for shotguns in my experience. Most people shoot HFT with approx 360mm LOP as a rough guide, and maybe 10 or 20 mm longer for FT from the guns that I have measured.
It could also be that for your BSA R10 HFT rig that you have a more normal scope height , and that then makes you hunch over when sitting and trying to look through the scope. I tried setting up a rig for FT and HFT and came to the conclusion there are too many compromises, and it really needs another set up to be completely comfortable.
You can do it but you will have to compromise and decide what you want it set up for, you could also try adjusting your butt pad slightly lower rather than extending it too much. That might help by lifting your head more in a sitting position.
Last edited by DEAN C.; 9th May 2016 at 11:22 AM.