Hi David,
1.)
I see your reason why you use 2 potentiometers in parallel: OK.
2.)
I reckon the accuracy of your measuring devices are too bad.
Just consider the direct Ohm measuring of the AD8400 resistance with your fluke 117:
In the 0 - 600.0 Ohm range the accuracy of it is stated: +-(0.9% + 2Digits).
Your measured value is 112.5 Ohm --> your uncertainty is +-(1.0125 + 0.2) Ohm = (2 * 1.2125) Ohm = 2.4 Ohm.
Now you say that 0.66 Ohm equals to 1.7°C --> 2.4 Ohm equals to about 6.2°C.
And this is just the calculation for 1 measured value! Your measured voltage drops (e.g: 203.7 mV) also have
uncertainties...
What about the accuracy of your measuring device? Is that device reliable?
3.)
What value do you want to get most accurate? The temperature value shown by your temp. measuring device?
Best regards,
Reinhard