I draw tick marks every 4 mins (one thumb width) along my track, then label every 20. Then if you are at about say 16 minutes, you should see the feature under the tick mark for 16 mins. If you have a head wind you'll be late, tail wind early, cross wind slightly off track, but only slightly for each tick mark. No big deal if you miss a couple either, you'll pick up on the next ones, the timers running.
This was taught to me by an ex-harrier pilot who did this sort of thing at 400kts 200' above the ground, and it works well. I still prefer navaids, but as a VFR navigation technique it is pretty good.
Cheers
EA
