Re: Magpul UBR problem
+1 on what ORD says.
I have a UBR with the 0.7" recoil pad. I believe the LOP is greater on the UBR than the PRS?
With scope eyepiece placed level with the CH, I need the UBR fully extended to get correct eye relief in the prone but my cheek falls comfortably on the cheekpiece and doesn't fall into the pit left by extending the butt (which is, if I understand correctly, the problem with your set-up?).
I'd suggest removing the optic, getting the LOP sorted so that you have a comfortable position on the trigger/pistol grip/cheekpiece then remount the optic at the correct eye relief....even if that is a little further forward and doesn't "look" quite right.
If you're large/tall and need the full extent of the LOP to get comfortable, I'm thinking that the SAPR from Battleline Inds would help bridge the "pit" once it's available for Magpul?