I had the same problem when I first got my TAC 15.
jon.henry755's advise is right on! You will need two sets of the Signature Zee Ring Pos-Align Offset Insert Kits to get the proper adjustment. Each kit contains three (3) offset inserts, one .005" offset insert marked -5 and +5, one .010" marked -10 and +10, and one insert .020" marked -20 and +20. You will need to use the .020" inserts, (one .020" from each kit).
Rotate the front insert so that the +20 or fat portion of the insert is down and install in the front ring. Rotate the rear insert so that the -20 or skinny portion is down and install in the rear ring. This combination will give you the max tilt of your scope.
If this isn't enough, you can move the rings as close as possible to the adjustment turrets for some additional scope tilt and movement for eye relief. This is what I did to solve the problem....see photo below.
You should have leveled/squared your crossbow before mounting the scope.
You should also move your scope windage and elevation adjustments back to center before mounting the scope.
You should also move your rest back to where you weree shooting bullet holes.
Hope this helps.