Gus - is the smile on your face caused by where Fern has had her hand?
Max drift is fine for heading calculations, but it is much more difficult to use for groundspeed calculation. Let's say that at 2000 ft and ISA, the wind is 50 deg off at 20 kts, that means a headwind component of 2/3 x 20 = 13 and a bit, call it 15 kts. If your IAS is 90, then that's a GS of 75. Now, the first fix on track is after 21 miles, so that'll take (21/75) x 60 = err, umm .....well, at 90 it would have taken 2/3 of 21 which is 14 minutes and we're doing 5/6 of 90 so it'll take, err..6/5 of 14 which is 12/10 x 14, so that's 120 plus 48 divided by 10 = 16.8 minutes.
Fine - but what a faff pre-flight. Yes, clever folks can do it, but why bother? It's within a minute of the correct value - but it seems more trouble than its worth to me!
Now can Gus please turn out the lights?