I read of a way to measure seat hight without having to measure any angle. Put the forward crank parallel to your seatpost (which will not be 6 o'clock, but maybe 7 or 7:30), and lock your knee. Now your foot, in the normal position on the pedal, should be parallel to the floor. This will give you the requisite knee angle when the pedal is at 6.
Of course, this doesn't help with the 3 o'clock position I suppose.