The following is an excerpt from a Sys 30 manual, the adjustment is in deed in the lower RH bezel screw.
I like the use of GPSS and watching the track line for centering.
4.7 Roll Centering Adjustment
The Roll Centering Adjustment
compensates for any offset that may exist
in the Turn Rate Signal, which
is internally processed by the
Turn Coordinator / Roll Computer. This
adjustment should be performed on the
ground, following replacement of the
Turn Coordinator / Roll Computer. It
should also be performed in flight, if one
of the following symptoms is observed:
1. Heading Bug is offset from the Lubber
Line, while holding a heading with the
heading mode engaged.
2. CDI needle is offset from center, while
tracking a course with a navigation mode
engaged (LO TRK, HI TRK).
4.7.1 Aircraft on Ground Adjustment
4.7.1.1 Preparation for Adjustment
a. Set Autopilot Master Switch to OFF
position.
b. Set Avionics Master Switch to OFF
position.
c. Set Battery Master Switch to OFF
position.
d. Set APU Power Switch to OFF
position.
e. Connect APU to A/C.
f. Set APU Power Switch to ON position.
g. Set Battery Master Switch to ON
position.
h. Set Avionics Master Switch to ON
position.
i. Set Autopilot Master Switch to ON
position.
j. Wait until RDY lamp alone becomes
illuminated on AP display.
k. Turn OFF Navigation Receiver to
center CDI needle.
l. Engage LO TRK mode.
4.7.1.2 Adjustment Procedure
a. Insert the Roll Centering Adjustment
Tool, PN 95101-1, into the bezel hole
shown in Fig. 4-2, until it makes contact
with the Roll Centering Potentiometer.
Caution: To prevent internal equipment
damage, do not apply excessive pressure
to the Roll Centering Potentiometer.
b. Adjust the Roll Centering
Potentiometer in small increments, until
A/C Control Wheel movement is nulled.
Note: Allow time between adjustments for
autopilot to stabilize.
4.7.2 Aircraft in Flight Adjustment
4.7.2.1 Preparation for Adjustment
a. Trim aircraft for level flight.
b. Intercept a course.
c. Engage LO TRK mode.
4.7.2.2 Adjustment Procedure
a. Insert the Roll Centering Adjustment
Tool, PN 95101-1, into the bezel hole
shown in Fig. 4-2, until it makes contact
with the Roll Centering Potentiometer.
Caution: To prevent internal equipment
damage, do not apply excessive pressure
to the Roll Centering Potentiometer.
b. Adjust the Roll Centering
Potentiometer in small increments, until
A/C Control Wheel movement is nulled.
Note: Allow time between adjustments for
autopilot to stabilize.