Well there are a lot of online “ohm calculators”. One example:
Simple to use Ohm's Law Calculator. Calculate Power, Current, Voltage or Resistance. Just enter 2 known values and the calculator will solve for the others.
ohmslawcalculator.com
But, not sure how that would help. The ohm readings you see are the baseline that sets up the formula on the device. If you have a good connection between the battery and device, the ohms shouldn’t deviate much. When I see a large difference, it’s usually a wanky connection caused by the devices that use the magnetic adapters.
Since you can’t manually change the OHMS on these inexpensive battery devices, it wouldn’t matter if you use volts or watts, the output is still tied to the baseline reading of the detected ohms. As I keep an eye on the ohm readout, I just adjust up/down based on the “hit” quality. Even when my ohms are consistent, I notice a vapor drop off as the cart gets close to empty, so I boost the voltage to compensate. Basically it’s all feel, no formula.