im usually using abs(angle(x1,y1,x2,y2)) that solves it for me most of times.
abs-olute basically takes "-n" and shows u "n" takes "n.nnnnnnnnnnnnn" and shows "n" absolute values to the rescue lol
Edited: however my suggestion it might create problems giving only positive values... R0j0's formula should work better.. makes more sense in mathematical way.