The answer is to do the simple mental arithmetic involved in approximating the rate to the nearest half knot or whatever. You dont want to be messing around with bits of paper in the middle of a wet and windy night.
As for the RYA teaching you a particular method - they dont. Your teacher might have been daft enough to do so but I hope not. The RYA advise you to find a method that works for you. Find your own route to salvation - use your noddle.