When doing the initial install before shooting and zeroing, I try to come close to centering the sights in the dovetail. To do with with calipers, I install the sight and measure the distance from the left edge of the sight to the left edge of the slide. Then I measure the distance from the right edge of the sight to the right edge of the slide. If the two numbers are the same, then the sight is centered on the slide. If one is larger than the other, you need to drift the sight towards the side with the larger number to close some of that distance. It doesn’t have to be perfect but getting it close to centered at least gives you a starting point for zeroing.
ETA: if you’re asking which part of the caliper makes this the easiest, I use the depth rod which is the part on the opposite end from the jaws. Open and close your caliper jaws and you’ll see a thinner point expand and retract on the opposite end.