Stay with the "or less" part. A` servo's torque rating is derived by using a distance of 1" from the output shaft. Distances outward of 1" from the output shaft are automatically reductions in the servo's useful torque. I sometimes use a 4" arm on a 35% plane and frankly, it's not needed. 3" works fine even on my ganged rudder servo installations. The one I'm setting up now uses a 2-1/2" arm for torque and deflection reasons.

On the Edge you shouldn't need to have a spread any greater than about 2-1/4 to 2-3/4" at the rudder horns. The same distance would be used at the rudder servo arm. After 7 of the 28-29% planes I've gotten it down to a science.