This about setting Geometry 6° (tuning) for a Roban EC-130.

The Collective Pitch range is set from -3° to 10°. The Subtrim (tuning) center stick pitch is set to 3.5°. For Geometry 6° (tuning), I increased the parameter to 128 and get a pitch of -1.5°, which is 5° from center stick. Should I keep increasing this parameter until I get a pitch of -2.5°, which would be 6° from center stick?

To measure 6° value correctly you will need to set servo center in the middle of travel - with 0° between main blades.
From then measuring 6° is possible.

Measuring with your way should be also possible, but not perfectly precise. However in the worst case you can just decrease the Geometry 6° if some odd behavior is visible.
Yes, getting to -2.5° in your case might be better. In any case if using a fuselage the settings could be even different because whole model has bigger mass.
For example if you can see oscillations at aileron during slow descending (similar to autorotation) your Geometry 6° is already too big.

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Spirit System developer