Battery Calibration Off

I am trying to calibrate the voltage readout on my OSD. I use a 10,000mah, 4s battery and ideally, when the readout says there is 20% battery remaining, I should have used 8,000 ma and have 2,000ma left. The battery is only using about 6500ma, (the amount of recharge), I’ve tried the by-the-book calibrations and still get the same figures. What adjustments can I make to get the readouts right?