WW - good idea! I do something similar when mentally replanning whilst airborne but use 6 min increments. As six mins is a tenth of an hour, the maths is easy:
... at a g/s of 140kts, I'll expect to run 14nm in 6 mins or 7nm in 3. My next turn is 20nm away, I'll be on the turn in 9 mins.
A s/hand whizz wheel and a secondhand or homemade rule and protractor work just as well.
Stik