Or you could try this:- “Factor” for 75kts is 0.8(time taken in minutes to complete 1nml)
The factors for each groundspeed are printed on the Airspeed /Groundspeed plate of the NavOK equipment.
21×0.8=21-21×0.2=21-4.2=21-4=17-0.2=16.8mins. But the windspeed was overestimated therefore the groundspeed is a little faster than 75kts. 16-16.5 mins will do me. I’ll be looking out for the target at 10-12mins.