Wow, This is a lot of stuff that could be said much plainer.

Here is how I was taught to make the differential many years ago. Now, this is for a high-wing setup where the servo is UNDER the wing. The linkage arms are in the center and behind the servo, but you'll be able to make it work for your setup once you understand it.

We used a CIRCLE servo arm on a single servo. Consider the FRONT of the wing to be 12 O'Clock, the bottom to be 6 O'Clock. With the ailerons centered with the wing, bring the linkages off of the 2 O'Clock and the 10 O'Clock positions on the circle arm.

When the ailerons are run they will automatically have differential in them, the UP will be more than the DOWN due to the fact that the linkage goes mostly sideways going around the clock from 2 to 10:30 when pulling the aileron DOWN, giving less travel. Going from 2 to 5:30 you get a straight push UP and substantially more travel to the aileron. Thus, you have differential.

If you were running a low wing with the servo on TOP, then you'd use the 4 and 8 O'Clock positions. (That's not your setup, though.)

I just read Da Rock's post, mised it the first time. His is the formula to make your setup work right. I'd go 2 and 8, but 1:30 and 7:30 is certainly close enough to the same thing, you get the idea. Just make sure that the aileron goes DOWN less than it goes UP. Work out that motion before you cut it up.

Hope this helps.

Jim