ORIGINAL: Redback

Measuring throw in inches, millimetres, or miles for that matter varies depending on the width of the control surface at the point of measurement. Half an inch of travel will be different for different size aircraft.

A measurement in degrees however is the same irrespective of the plane, which is why the meter is graduated that way.

The magic formula is Sine A = D/W, where:-

Sine A is the Sine (trig function) of the deflection angle
D is the deflection
W is the width of the control surface at the point of measurement.

Or, as someone has already said, you could just guess

Terry