So it is better to be taller, causing the bullets fired at you to fall short, than be shorter and have the bullets fire over your head? I'd rather them open fire thinking I am further away than I actually am.
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.