The wavelength is calculated by 300 / frequency
432.650MHz (625 and 675 commonest on 70cm packet), equals about 69.3cm. Velocity factor means that the actual wavelength in terms of metalwork is nearer 65.9cm.
A dipole will be half that of course - but don't wreck the 2m antenna just yet - have you a wire coat hanger you can unravel?
The length of side 'B' is 3/4 of the above calculated wavelength.
The length 'A' is a half wavelength. finally the stub 'C' is therefore a bit less than a quarter wavelength - but there should be an air gap between a and c.
The feeder is attached at points X and Y which are simultaneously slid up and down (left + right on the above picture ) the shorter loop to get a good match.
This 'slim-jim' works better than a dipole - and wrapped with masking tape, it even works taped to a window.