like i said you use the same method for drift calcualation, but on your legs if you have a head wind and are slower you just recaluate your ETA. This is why for example on a 15 min leg you have a chx point 1/2 way (7min 30) and you then simply work out your new ETA. You also put away your map for the whole flight, only getting it out 30 secs or so before your next ETA and then refresh for what you are looking for. (as you memorise your route before t/o)
The whole system is geared towards low level navs where you cannot afford to be blundering around with a wheel/staring at a map every 30 secs.
Clock and compass are the most important instruments when doing a nav.