My shorthand for 90degree cant is this: hold towards the magazine well and "up" (skyward) to account for the path/drop of your bullet.
Example - if I wanted my POI to be the mouth of a target, I may hold on their eye, depending on which way I had the gun canted...
Depending on the gun/optics you're shooting, you may start to notice it past 50yds; 100yds is where both induced errors really start to become evident, IMHO.
Best way to deal with this issue is to live-fire it at the range. Get yourself a blank target backer, stick a clear aim point on there and try out different cants at various ranges, marking your groups after each run.