If I had dialed the elevation, the opp value would have decreased as the bore comes closer into line with the scope. And since the 15deg is constant, the adj value would've decreased as well, hence the mil values of the shift are decreasing.
Okay, the first thing is the 20 MOA scope base is not relevant to the point if the rifle is zeroed at lets say 100 yards. All the 20 MOA base does is offset where in the scope travel you will be when the rifle is zeroed... so lets get the scope base off the table because it has no effect.
As for your trig example showing the 15 degree rifle cant with the scope level, what you describe is almost correct with the 0.517 bore offset.
Since you are aware of this offset, you would zero the rifle so the POI at the zeroing distance is ideally 0.517 to the right of the point of aim.
From this point on the bore is firing parallel to the line of sight through the scope, but 0.517 to the right.
If you zero the rifle so the POI is at the center of the reticle, then you are inducing an angular offset relationship between the scope and bore and at twice the zeroing distance the bullet will impact left of the point of aim by 0.517.
If you zero the rifle at 200 yards, the impact at 400 yards would be left 0.517
If you zero the rifle at 400 yards, the point of impact at 800 yards would be left 0.517
If you zero the rifle at 800 yards well.... 1600
