I don't know that there IS any "calibration"...
Some sending units have contacts in them...rising contents in the tank completes the circuit on each one. The accuracy depends upon the depth the tank, 'cuz wherever the lowest contact is, is where the gauge considers to be the bottom....if it only reaches partway to the bottom, you won't get any reading--or an empty reading--till the contents rise enough to trigger the next level contact. If I recall correctly, the Wema senders are float, similar to the gas gauge in your car. The position of the float is indicated on the readout panel...and again, the depth of the tank determines how accurate it will be. If yours has quit working altogether, it's prob'ly clogged...Wema holding tank senders are notorious for clogging. You can try pumping out your tank and then filling it again with water and a little detergent....let it stand for a few days to give the detergent time to dissolve the animal fats that have clogged it up, pump out...and rinse well with clean water. If that doesn't work, you'll have to remove it to clean it.
