@Gyro @Kleinstein many thanks for all the comments and thoughts.
To stop the confusion, I relist those, so we can refer to the items more accurately
A 1:1 auxiliary arm accuracy (for 2:1 hamon trimming)
A.1 trimming with 6.5 DMM loading, bias current, leakage current to ground
B Hamon method error
B.1 Method itself,
B.2 10:1 vs 2:1 deviation due to voltage or power
B.3 Contact/switch resistance
B.4 EMF from the contracts
B.5 Trimming error due to again DMM loading, bias current, leakage current to the ground
C Hamon divider resistors
C.1 resistors TC and drifting
C.2 10:1 vs 2:1 deviation due to voltage or power
C.3 more voltage or power on resistors, deviation due to voltage or power
for A.1 B.5 trimming, I am planning 9V battery + 6.5 DMM,
so bias current impact (max 50pA, for 10k load, the worst case would be 500nV maximum) can be observed with 9V off, it can be checked and compensated,
regarding loading, DMM can be set at high impedance mode (10Gohm), so for the impedance 10k resistor range, this error is considered to be sub ppm
leakage current, the whole system would be floating, so it can be ignored?
for B.2 10:1 vs 2:1 deviation due to voltage or power
the resistance would in 100k to 10k range, so the power on the resistor would be 10mW or 1mW, so hopefully those can be designed out with proper hamon resistor sizing. they are all low PPM resistor + select input voltage carefully + fan maybe
for C.3 power or voltage deviation
I googled and my understanding, VC more obvious for resistor more than 100M, so sub PPM again if the select hamon resistor less than 10M?
the devils are all in the details. Surely, I will find out more when I start building them.
With all the assumptions about possible errors, I am not that confident without checking the end result (maybe some bridge method)
thanks again for sharing your thoughts and happy new year.