Joe, if you look at the drawing I put up of the control linkage analysis, and look at the middle section where I show the control surface at neutral, you will notice that I drew in a heavy line showing the control link vector at 90 degree angles to the servo output arm and the line from the control horn connection and hinge. If you do this, there will be almost no differential built into the linkage (due to the unequal lengths of the input link (servo arm) and output link (horn) there will still be a negligible amount).

The actual pushrod in this case has a little bend in it to help clear the slot in the wing, so I did this drawing to get the measurements right since it would be nearly impossible to eyeball it in this situation. CAD skills are not absolutely necessary. You can do it on paper with a pencil, straight edge and compass too.

Back in the day when I was flying slope gliders before computer radios, we used to use mechanical differential in the elevator to produce more down than up. Since the elevator also functions as "throttle" on a glider, at high speed (nose down) you need less sensitive elevator to slow down. Nose high, you will be flying slower and will need more authority in the elevator to put the nose down. I have yet to experiment with differential on my pattern plane. I am still learning trimming and may use aileron differential. I will have to get my scores to a point where I really notice any yaw coupling first. I am also wondering if there is any reason to put differential in the elevator in a pattern plane.