As oft explained (and you can explain to yourself using a piece of paper and a ruler) for all practical purposes crossover is mythical. Cant, on the other hand, is not. It's quite shocking, when first installing a spirit level to a gun, to note just how completely and utterly useless we are at judging the vertical when one's head is held at an angle wrapped around a stock - indeed, the vertical feels really "wrong".

Not to forget other things that may come into play like PX error, and zoom-wandering-zero error which can all conspire to change POI at different distances.