Airport fuel pumps are calibrated by law, so there is no need to drain a tank to empty, just fill to a known point (fuel tab) or to full, go fly, come back and refuel.
You should have an fairly accurate consumption rate for that flight.
You can then compare this against what the fuel computer is telling you and then calculate the K factor.
Do this over enough flights and you can develop an accurate value.
To calculate K factor;
(computed fuel used / actual fuel used) * your current K factor.
Use this new value and do it again until you reach a resolution you are happy with, in other words your computed fuel consumption matches what you have put in at the pump.
Cheers