This is how I set my symmetric shifters up on my old Miyata 1200, and the trim function always was perfect.
The Suntour Symmetric shifter was a unique and clever piece of engineering from Suntour. IMO, one of the coolest and best friction DT shifters.
To get the most out of the trim feature, the shifter cables need to be properly adjusted.
1. First, have the right shifter positioned for the smallest cog of the rear wheel.
2. Tension the shifter cable to remove any slack that would cause slow response.
3. Put the right shifter to engage the largest cog of the rear wheel.
4. Place the left shifter for the small chainring, pushed all the way forward.
5. Loosen the FD cable stop bolt, adjust the low trim screw to have no chain/cage contact noise from this low gear position.
6. Now tension the FD cable, remove all slack and tighten the FD cable stop bolt.
Now the auto trim function will provide maximum movement when the right shifter steps to higher gears.