Typically, a Range Official will "swipe" the (50mm wide) narrow side of his scoring overlay between the grip and the competitor's torso. The "grip" is the part of the gun which is "gripped" by the strong hand, so from just below the beavertail to the end of the frame seems reasonable to me.
However the 50mm rule was never, and is not, intended to be a precise scientific measurement. The rule originally alluded to concealability, and it was written to prevent blatant violations whereby the handgun grip would stick out like an unmentionable part of a dog's anatomy!
This is why Rule 5.2.5.2 gives the RM leeway to allow for "anatomical considerations", and I'll usually allow up to 10mm extra.
In my experience, skinnier competitors have more trouble complying with the 50mm rule than Fat Bastards® like me, who intentionally develop and maintain "love handles" purely to assist with rule compliance.
This how Mr Pinto applies the rule re: distance. It's at the RO's discretion.