Found the answer in a book
all sorted now thanks
So MD= TAS/miles per min.
eg. 240kts = 4 miles per min
therefore if you have a wind speed of 40 knots divide 40 by 4 = 10 degrees max drift
MD at 45 to 90deg is 10 degrees and so on and so on...
SJ's
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Catch you on the flip side!