Apparently the RAF can only navigate when flying at multiples of 60 kts.
Bulldog 120, Gazelle 120, Tucano 240. JP 240 or 300, Hawk 420. It makes the mental maths very easy. The exception to the rules where the few that were multiples of 30 such as the Chippy and Wessex (90) and the Tucano in the Nav training role - 210. This last one was to allow the aircraft to catch up if it was late. The Tucano could only relay manage about 255 at max chat on the flat.