At the twelfth fret, John. Check that the "open" and "fretted at the 12th fret" note are exactly an octave. Use an electronic tuner if necessary to ensure spot on accuracy.

If the fretted note is sharp of an octave compared to the open string, the distance between the bridge and the 12th fret is too short, so the bridge should be moved towards the tailpiece.
Etc Etc !! Trial and error!

Oh, and put a small pencil mark on the banjo head just to mark the position of the bridge when you have got it right, it'll make it much easier to repeat the process in future.