Hi, if you assume that magnetic = true for the purpose of an easy calculation, then you can use a velocity triangle to determine the wind velocity. The Hypotenuse side is your airspeed, and the Opposite side is the wind speed. Remember OHMS? Opp/Hyp means Sin.
So for 30 degrees, sin 30 = 0.5
Which means your cross wind is 0.5 of your airspeed... Say 45 knots if you are in a 90 knot cessna.
(Even allowing 3 deg magnetic variation, its still 41 knots.)