The gas gauge is a little volt meter. Very small current. So, not much to move it. The problem sounds like a bind in the gauge. Might be some dirt or it might touch the face of the gauge. I have seen both. You can get it out. But it will take a new gasket for the glass front. It is working. Just not correctly. Should have gone to Empty when grounded. Full when open is just right. The idea to set the 1/2 tank is a good one. Look close at the needle and see if it rubbing or sticking on any thing. A very small spring is on there to get it to empty when key is off. I hope that helps.
