Are you refering to the gauge on the actual tank, or your inside monitor? The tank can only be filled to 80%, so the gauge on the tank will (should) only reach that amount.
I adjsuted it so that when the tank is at 80% (as full as it should ever get) the level light inside shows 100% Watch it as you use up the gas, it should show empty about the same time it really is. Would be nice if you think you have some and be out and it be cold enough to need to run the furnace or want to cook.