Perhaps my English was not clear..... Please aknowleadge that I can not adjust the servo center mechanically because it's impossible to center the servo arm.
Pratically, your suggested METHOD-4 is IDENTICAL to my METHOD-2 (even if explained in different manner), therfore the solution is: adjust servo arm center position via Powerbox.
I would like to receive a more articulated reply from your side. Ok, thanks in any case