Mike, excellent illustration on obtaining mechanical differential throws.
IMO, this is basically an application of what some have termed Ackerman differential. With known angles and lengths for the servo arms (and control horns if other than 90 degrees) the differential can actually be calculated. IIRC it's a cosine function. While the actual math isn't shown,
http://www.qmfc.org/school/ackerman.htm illustrates this in a slightly different manner (in terms of pull-pull systems), but it's still applicable here.