I didn't think that the 20 had canted hulls, but I am having the same thoughts with my infusion which certainly has canted hulls.
When you are measuring the difference between the front and back edges of the rudders, it is important to imagine a line across the blades which is parallel to the waterline. Then measure at either end of this line. This is true for all cants of hull.
it always amazed me how good the tiny rudders on the 20 were. I used to run with a little toe in. What setting are you aiming for

Paul