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For anyone who may be interested in having the cable travel adjustable, I'll share this idea...
Say you're using a 15mm servo horn, but don't all gears covered? You can use a larger diameter and try it out of course. 
In the process of developing Shift4Me, I thought about an adjustable travel of the cable, with the same servo horn.
Here's what I came up with: a piece that would turn around a fixed point of the bike, and with cable fix points that could be adjusted along the piece. That way, you could adjust the amount of travel without disassembling anything.
I never tried it out (because I loved to keep a minimal design) but it should work.
Possible disadvantage: a little tolerance in the travel caused by the pivot point of the piece.

This is going to result in quite a bit of nonlinearity. On the other hand, this is might be a good thing actually! You want more servo leverage at lower gears, when spring tension at its highest... I think a cam-shaped puley specifically designed for amount of cable pull your shifter has will work best. You, basically, need two separate designs - most shifter systems are either close to 2:1 like old shimanos, or 1:1 (sram, new shimanos).

Campa is inbeetween.

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Using a small stepper motor plus a screw actuator like on x-shifter is the best setup actually - built in friction, more precize... but more complex.