Even your multimeter cant reliably measure to 1mV unless its very well and recently calibrated and high quality. Even the different metals in a test lead connector can generate a couple of mV.
I have one of these...
https://www.fluke.com/en-gb/product/ele ... /fluke-289And with a recent calibration certificate. It gets close. But it needs to be on, in a temp stable room for half an hour before its accurate to 0.025% of its scale +/- a few digits on the end figure. And it costs the same as 2 PL8s and then some.
Those cheap cell logs, are like buying 8 voltmeters for a manufacturing cost of a few pence. Theres a BIG difference between resolution, and accuracy.
You only need them to show a drop at the end from 3.2xV down to say 3V so as long as you are only using them as an alarm to stop battery damage theres no problem The danger is that people insist on thinking that they can monitor a battery like a fuel gauge based on volts. You cant.
If you wanted to try, then you would need to do these things.
MAP the voltages, under a 1C discharge, and a 2, 3C discharge under discharge conditions with a new battery, and at various temperatures. Then do the same in stages with zero current, breaks so as to have an unloaded voltage graph on discharge. So about 20 seperate discharge curves and graphs.
Then...
Fit a temp sensor to your battery. And use a computer to monitor current, and voltage, and temperature. And with a little averaging, and a bit of maths, you could have a fuel guage based on the stored maps. Thats the ONLY way to do this. And that will notwork for lead. And will be completely different on every different type of lithium ion phosphate battery depending on the exact chemistry and percentages of each chemicals, types of anode/cathode etc used. So it will be individual to YOUR pack.