Whilst measuring drift angle, it could also be used for a ground speed check by timing, in seconds, an object on the ground, or a wave on the sea transiting from one graticule to the other and using the computer on the LH side.
The orientation of the graticules on the driftsight assumed that the driftsight was looking below the aircraft. On the Beverley the driftsight was not level in the horizontal plane but tilted slightly above the horizontal so that the side of the aircraft didn't interfere with the view. Hence one could get an accurate drift reading but the groundspeed was unusable. This made navigation even more 'interesting' as we had virtually no navigation aids.