RE: dual trottle servos
Expecting perfect tracking on two same brand servos is a bit much. The servo travel depends on several factor, the most prevalent being the degree of match on the feedback pots and the two resistors usually found on each end of the pot. These are typically 5% or worse tolerances in the parts used. The price on the servos would skyrocket if they were required to be precise within 1%. What is amazing is that they are usually so closely matched, not that there is often a difference. If it becomes especially important for your use, the easiest way is to try several on the bench and pick the two most identical or, open them up and tailor the resistors.