An alternative, should you wish to experiment is what I do, which gives me separate trim buttons for rudder and nose wheel. The rudder trim button is the normal one below the rudder/throttle stick, and the nose wheel trim is the "eyebrow" button at the top left of the rudder/throttle stick. (I'm assuming Mode 2 here.)

Plug the rudder servo in to the normal RUDD channel and the nose wheel servo in to one of the AUX channels. I usually use AUX 5 or above.

In "Device Select", set the AUX channel used to ACT and L. TRIM.

In "Wing Type", do NOT select anything for RUDD, should be INH.

In "PROG. MIX<whatever>", select RUDD as master and AUX channel as slave. Set Pos. 0 to L 100% and R 100% (or whatever floats your boat.) For Pos. 1 set L 0% and R 0%. Finally, set GEAR to "1".

With this set up, you can trim the rudder normally and independently of the nose wheel. Additionally, when you retract the gear, the nose wheel servo is turned off and centered.

Have fun!
...jim