Well., are you asking this as general curiosity, or for an AK build, a bad mag, feeding issues?? It depends.
The mag latch on the trigger guard seats the magazine at the rear tab. Since the front of the cartridge is a smaller diameter it can clear the bullet guide and be guided into the chamber.
Normally the left top area of the magazine feed lip will seat just under and left side ejector rail and this sets the mag height. There is some variation based on the design, wear etc.
An out of spec mag may have bulged or damaged feed lips, a bad follower or other defects that effects feed.
It is kina., a mag feeds or it doesn't. Inspect it for damage, bent feedlips etc.
If multiple mags do not feed then start looking at the other areas like the mag latch etc.
There are specs for some of these measurements., but generally meaningless to the average user and without some specific measuring tools etc.