Sub-trim and handset trim are different.
Sub-trim moves the neutral of the servo and maintains even throw in each direction (within the mechanical limits of the servo of course)
Handset trim moves the neutral but the trim end points remain the same, so as you move the trim to the right, you will have less right lock to play with and vice versa.
Use sub-trim to set up the linkages on the bench, use handset trim for fine tuning on the track. You should never need more than a couple of clicks of handset trim, if you do something is awry in the chassis setup.
HTH
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