I'm not an instructor, but do fly around a lot of aircraft with uncertain performance.
I always use the distance marker method - usually 6 or 12 nm markers, depending upon the speed of the aircraft - but any interval will do so long as it's constant. For on the hoof planning, it works wonderfully once you've grasped that airspeed is irrelevant. If you know that you're flying (say) each 12nm interval in 8 minutes, you don't worry about the fact that you're flying at 80 kn, you just use your 8 times table. 5 markers to go = 40 minutes. Cuts a lot of mucking about out that detracts from lookout, managing the aeroplane and spotting landmarks. I find the method, even in a microlight at 40 kn is good for 1 minute in an hour when predicting an ETA.
G