Ideally the mechanical setup provides equal travel volume in both directions. This is achieved primarily by adjusting the control link so that the servo arm angle is centered in the travel arc at surface neutral.

Unless you start at a “0” neutral value the values will not be proportionate to achieve equal travel arcs. Even with a “0” reference point you maybe off slightly due to electromechanical tolerances of the servo in play. If your servo arm needs to be programmed for -10 to match the required servo angle for equal travel arcs you have an offset in play. So your numbers will be skewed in the direction and number of the offset.

-10 is the zero reference point
-10 + 130 = -140L
-10 + 120 = 130R

In either case you value is 130.

I’d recommend programming the dead-band to the lowest value as long as you pay careful attention to linkage binding and current draw at neutral and extreme travel.

H9 and others offer inline current and voltage meters that plug into the servo connectors which are invaluable IMO for determining a proper setup.