I have a lot of difficulty with this, and I think a few additional features would help:
when a co-ordinate is selected, its position displayed somewhere, preferably in a small box next to it.
keyboard nudging of the selected co-ordinate point. Useful when the bounds of a particular zone need to be very precise.
with a point selected, the Tab-key will select the next one, clockwise. Shift+Tab; anti-clockwise.