1.7m is a common height used for Russian optics of military weapons, used to measure distance based on the height of a person in the scope.
https://en.wikipedia.org/wiki/PSO-1#/media/File:Distance_mea...
https://en.wikipedia.org/wiki/PSO-1#/media/File:Distance_mea...
You aim for the center of mass -- roughly at the heart. A bullet, thusly aimed, that drops slightly short can still hit someone in the leg or lower abdomen. Or it'll hit the ground and kick up dirt, be more visible.
A fixed height for each tick mark also means you can use the enemy high to roughly calculate distance.