From another similar thread: "Let's use your rudder @ 20 degrees as an example. Measure it at the widest point. Say this dimension is 3". Multiply by two to give you the diameter if the rudder could move in a full circle. (6"} Multiply this times pi (3.14). Divide the answer ( 18.84) by 360 and you have the length of 1 degree. ( .052"} This times 20 is 1.04" throw for the rudder. You will be measuring a chord and not an arc, but it's plenty close enough."