Will someone enlighten me as to what happens in FSX?
I set the wind at 090 degrees at 10 knots.
I realise that FSX uses wind true not magnetic.
From the sectional the magnetic variation is 13 degrees East
Therefore I would expect the magnetic setting for the wind would be 77 degrees.
However, when I press SHIFT+Z it gives the wind as 71 degrees magnetic at 10 knots.
Am I missing something?
Not really, wind isn't really a constant velocity - it's prone to small unreported changes...the 6 degree difference makes minimal difference and could be expected in the real world too, providing it isn't constant and is only a small variation.
Other than that, the method you used for calculating magnetic from true is spot on.