Speedhut Gas Gauge Manual Calibration

They have several presets, but they are the same ohm range for the NOS sending units. I have a new sending unit and my gauge is near death anyway so I will be going to a new one. I will call them I guess. It's just not clear if you have to set the full and empty in that order as in the instructions or if they are independent and that it doesn't matter if you set full today and then empty when you ran out of gas a week from now. Otherwise you'd have to lose a tank of gas to set them up correctly which I can't imagine.