Make this easier. Boresight from a benchrest at the range or at home with the rifle in a support of some kind. Center a conspicuous object about 100 yards away or so in the bore, then adjust the scope to the same place (the adjustments are backward from those used when shooting - if the cross hairs are to the left of the target, adjust the scope to the left, same w/ vertical adjustment.
Then shoot at 25 yards (or 100, if you feel lucky - I usually do this), refine the zero precisely.
While you don't need a collimeter to bore sight a rifle, they are useful for this and other things. Very easy to test the scope's adjustments, the security of the mounts (any slight movement is obvious). Important safety tip - take it out of the barrel before shooting.