Yogi:
At last someone has outlined the simplest method of mental calculation which I have always used. Any more accurate than that and perhaps you shouldn't be operating if the crosswind is that critical.
It also works for mental calculation of degrees of drift when airborne when you apply it to the maximum drift (which you calculated before departing:60 divided by TAS multiplied by windspeed in knots). The remainder (i.e. 1 minus the relevant value calculated from the clock code) when applied to the windspeed in knots will give you head or tailwind component.
For example:
Track 360 True, TAS 120 kts
W/V 300/25 (true). Max drift 12.5 degrees
Drift from the Clock Code:
60 degrees off = 0.9 x 12.5 = +/- 11 degrees: Heading: 349 T
Headwind: 0.1 x 25 = 2.5 kts HWC : g/s = 117.5 kts
haven't checked it on the whizziwheel, but seems to work in the air.
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"Take-off is optional, Landing is mandatory"