You really don't want the servo arm to be at 90 at neutral with a 401, which is like the biggest myth in this hobby. The 401 needs intentional differential on the servo arm to compensate for its lack of independent limit settings for left and right rudder without binding on either side while still providing full pitch range. You just want to be sure the differential in in the correct direction (otherwise you'd be better off with 90 degrees). This also somewhat allows (in the ballpark) normal mode mechanical centering and moves the slider through TR zero degrees (the dead zone) at peak linkage speed. Alternatively, with a 90 neutral setup you either have to give up centering in normal mode (which could be bad), a lot of right rudder authority (which is bad) or else allow binding (which is seriously bad).

Since the gyrodetects yaw rotation rate and dynamically responds, it doesn't care if the linkage is squared up with the arm at neutral. It's more important that it can deliver full pitch range in either direction without any kind of binding and with good servo resolution throughout the range.