I eyeball the angle. I know the distance between a stretched out thumb and pinky finger on my hand is about 17cm and use that to eyeball the distance.
-Kevin
I get 25.4 cm (10 inches) from my pinky to my thumb.
I used to use the forefinger to thumb measure for 17cm with my Oktavas on a AT (I think) t-bar. Just about right on for me, but now I use my handy custom made t-bar with holes drilled at different spacings with my AT933s. I carry a protractor for the angles (actually been teased for that) but now I'm familiar enough that I can eyeball the 90 or 110 pretty well and only bust out the protractor if I change things up or for verification.