Maybe this has already been said in words I don't understand, but if not, please consider this from a Hoosier hayseed:  With no other outside influence but gravity, our rifle is aligned with the target, and corrected for bullet drop with the appropriate sight ELEVATION.  As we cant the barrel (rifle) we reduce the elevation correction until at 90 degrees we have zero elevation correction and a whole lot of what is now un-needed windage (horizontal) correction.  The result, to my mind, is a bullet strike low and in the direction of cant.

How far out and how low to predict the bullet strike is about the only use I can see for mathematics.

To recover from this horizontal position, from the shooter's point of view, it's most simple to rotate the rifle in the opposite direction using the bubble, in a spirit level appropiately affixed to the rifle?

The problem I see for the shooter who disregards barrel cant as a sighting problem is that on turkeys or rams, especially, due to their size and distance magnifying the cant sighting error, the spotter is not going to be able to give an accurate correction to move the strike onto the target.  It can be argued that the shooter will continue to cant at the same angle, offsetting the error, but, since he is using no method to help him assure that the error will be the same, he will not be consistant.