I wish I had an answer for the inaccuracy issue. Unfortunately, the gauges are nortoriously inaccurate. We use ours as a rough indicator only and rely on the hour meter and our consuption rate. I can tell you that we have motored many hours past the point at which the gauge says empty without running out of fuel. (I don't recall ever adding more than about 32 gallons to the tank.) I suspect Espar can give you an average burn rate, which is likely very low in any case. (We run a Panda 4.2K generator that only uses 1/10th of a gallon of fuel an hour.) One thing we do is keep a 5 gallon jug of fuel in the life-raft revetment and add that once we've gotten down to indicated empty, just to be on the safe side.
Adding fuel at sea can be a bear so I carry a tube with a little squeeze pump, similar to the pump on an outboard. We heave too, steady down the yacht and for all intents and purposes siphon fuel into the tank through an in-line filter and water separator. Thus far we've not had a problem.
"It is not so much for its beauty that the sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."