Thank you Notso F.
MDR: Mental Dead Reckoning.
I know that the rule is a bit rough, you could however use 20 instead of 30 when the wind off is less than 50 degrees - still its safe and a bit conservative... The example I gave was not very good and the result repressents the XWC if the wind was 30 degrees off
(sin 30 x 20 = 10 kts XWC).
My mistake - sorry.
errare humanum est.