I used the "ruler on a mirror, open one eye, center it on the end of the ruler, close it and open the other eye, mark where the center is on the ruler, then rea

I used the "ruler on a mirror, open one eye, center it on the end of the ruler, close it and open the other eye, mark where the center is on the ruler, then read the measurement (in mm)" method. Did it a few times to be sure, exact same results every time, just have to keep your head still and geometry does the work for you. Glasses made using that measurement have been entirely fine, no problems.