Discharge current is a MAJOR factor in capacity measurement, especially with old cells that may have developed high internal resistance.
For example: 2 cells with ‘original’ 2000mAh capacity: #1 that has been ‘hammered’ for 5 years in a laptop, #2 used occasionally from new in a flashlight and never charged beyond 4.0v, or discharge below 3.2v.
If you discharge them both @ 0.5 amps you might get : #1 = 1500mAh, #2 = 1800mAh. Different, but not a major discrepancy.
If you discharged at 1.0A you might get: #1 = 500mAh, and #2 = 1500mAh. Noticeable difference.
If you tried to discharge at 2.0A; #1 might fail immediately, and #2 might go 1400mAh. Ah, now the difference is stark.
Then, if you checked the voltage on #1 it might still be 4.1v on rebound. #1 simply could not deal with the required draw anymore.
Any charger using sliders to measure internal resistance is inaccurate due to the resistance of the sliders. It’s like trying to do precision measuring using an old wooden stick with hash marks on it. It’ll do some kind of a job but it’s pretty crude.
Internal resistance/Impedence
If you are going to put that much work into this project, I think you need to minimally use the likely discharge current that the cells will have to deal with on maximum draw. I’d recommend trying to do at least 2x that if not more to weed out weak cells. Cells that are already old and weak, making the grade….barely……may come back to bite you.