Should I do low mount, medium, or high ?
Height of rings will be dictated by the diameter of your objective bell. Best height will provide a firm, but comfortable cheek pressure on the comb of your stock. Height of rings does not affect elevation on target but it is imperative that your scope be parallel to the bore of the rifle or you will run out of adjustment as you have in this case.
Based on the picture posted, your scope appears to be pointed "downhill", which would cause problem of running out of adjustment/high impact on target. Raising the front end of scope will lower POI (as will lowering the rear end).
The Signature rings allow for aligning the axis of the scope with your rifle bore by selecting proper thickness and combination of inserts (.005, .010 and .015"). For example, using two .015" inserts (one in front ring and other in rear ring), you could obtain up to .030" more height of the front of your scope which is probably more than you need.
Offhand, I'd try a .015" insert in the front ring w/thick part down. This would raise your cross hairs (and lower POI on target). May not be quite enough but using a .005 in rear w/thick part up could then be added. (Rings come with two sets of 0" inserts)
First, I'd find center of tube by counting clicks starting at one extreme or the other, then count half that number and you'll have equal amount of elevation adjustment ^ or v. Then, use the correct inserts to bore sight the rifle. This should get you on paper @ 100 with plenty of adjustment left on your scope.
Good discussion as to how to determine how much shim you need here:
http://www.predatormastersforums.com/forums/ubbthreads.php?ubb=showflat&Number=1103505
Regards,
hps