Cast diagram confused

help! i know this is a very stupid question but i can’t ask my teachers now i’m almost at the end of alevels 😭 when you get an angle (eg = pi/2) that falls ONTO a line in the cast diagram how are you supposed to know/decide how many OTHER angles there are left/ what these angles are as you cant so that symmetry reflecting triangle thing as it’s a straight line not an angle at a certain degree so no triangle…