I have a no brand (literally completely unmarked) flip up BUIS that I'm still trying to figure out and get zeroed.  Can anyone tell me what the A2 rear sight aperture's height is-or more specifically, the height of an A2-type carry handle rear sight-from turned all the way down?  The only way I could accurately measure this would be from the underside of the sight, as I have no way to find and reference any other datum.

The aperture in this BUIS is adjusted with a screw through it.  To adjust it, the screw must be loosened, the piece with the aperture moved, and then the screw tightened again.  It's obviously a PITA-no wonder the thing was cheap.

So anyway, to get somewhere near zero I need to adjust this thing by measuring where the aperture should be and moving it there.

Here's a picture of it:

Any clue at all?  I've used a caliper to measure where the large aperture on my carry handle sight is and matched the BUIS to that, but I haven't had a chance to try it out.