Mike,

Don't know if you know this, but you can "center" the servo using the servo arm. For example, if you have a servo arm that has 4 arms, 90 degrees apart, every time you take that arm off the servo and move it 90 degrees, you will change it's position by 1 output spline. So, if you keep changing that arm 90 degrees, at some point one arm will be at 90 degrees to the servo, or pretty close. Then adjust your other servo, using sub-trim, to match the first servo.

Ed