SoSmellyAir

I empathize with the OP because I also have mad OCD. So:

(1) To mount both STI levers at the same height, I stretch a ribbon (a) between the ramps of the handlebar and (b) resting against both STI lever band clamps, and I eyeball the ribbon to make sure it is perfectly parallel to the top of the handlebar.

(2) To mount both STI levers at the same rotational angle, I measure the distance from the brake lever pivot of each STI lever to a point at the transition between the top and the ramp on the opposite side of the handlebar.

Because of my mad OCD, my eyeballing is pretty precise. But before I install the bar tape, I ride slowly around my community to make sure that both STI levers feel the same to me. Now of course it goes without saying that all of the above first requires the handlebar to be perfectly centered relative to the stem, and for the stem to be perfectly aligned with the front wheel.