Calibrating Extruder on Ender 3

[TheOneLasagna 0]

I have been having problems calibrating the extruder on my Creality Ender 3 printer. According to this webpage(https://mattshub.com/2017/04/19/extruder-calibration/), you must measure 120mm of filament from the extruder and mark it, extrude 100mm of filament, multiply the current steps per mm value by 100 and divide it by how much filament was actually extruded to get the new steps per mm value. I tried this, and say if I had 20.5 mm of filament left and did the aforementioned steps, my value would end up overshooting it, and I would next end up with 19.5mm left. I don't know what the problem is, as I have checked and my extruder gear and it seems to be working properly and not slipping. It could be that I am not measuring the filament in the exact same spot each time, and if so, how do I? I am overall goal is to achieve a printing tolerance of 0.3 mm, as my printer has barely gotten a 0.5mm on a tolerance test and nothing more than that. I have also tried printing cool, unifying speeds, and controlling acceleration. Any ideas?