Back Azimuth = Forward Azimuth + 180° For example, if a line has an azimuth of 75, its back azimuth would be 255°. If a line has an azimuth of 150, its back azimuth would be 330.

STEBBING, F. Azimuth Tables for the Higher Declinations (limits of declination 24° to 30°, both inclusive) between the parallels of latitude 0° and 60°, with Examples in the use of the Tables ...