With practice the human eye is very very good at calculating angles.
Practice on scap till you can see the difference. At Bowman we had to
develop the ability to file both rod and sheet to a perfect 90 degree
fit on multiple pieces. they would be grouped at random, and if
pressure at the end of the row of pieces caused them to buckle at
all, you tired again. You eventually master the file, the feel and
the eye, if you are determined enough to keep trying.