The datum compass did indeed read in tenths - but it had uses beyond RAF Compass swings, in surveying.
The datum compass also required scheduled calibration and at Changi we used to swing the Watts Datum on an open area that doubled as part of the golf course. One day the inevitable happened and it was hit by a golf ball. I won the draw out of the hat for the trip to the Admiralty Compass Laboratory in UK to have it repaired - which gave me two weeks "leave" while it was returned to serviceability - one week at home and a second week in the "Route Hotel" at Lyneham, waiting for a flight. I promoted myself to commissioned rank during my stay, as the food was so much better in the officers' dining room and they had free newspapers in the lounge: nobody from the staff seemed to notice, and if they did they never said anything.