Have you considered doing the calibration with the sender out of the tank? The Maretron TLA adapter not only calibrates the tank level but also the level sender. On my last boat for the ease of doing the calibration I purchased two new senders and set them up in a jig simulating the tank profile. Then doing the level vs gallons calculations like you did I simply positioned the float in the jig and used that as the calibration point. It was easy and fast and eliminated the need to drain the boat's fuel tank... Just a thought.So, I wanted to try and calculate the affect of the inboard side of the fuel tanks being slanted on my fuel gauge readings. Not being close to the boat at the moment, I just drew an estimated tank which is geometrically a "right trapezoid" shape. Volume of my experimental paper tank is very close at 26.667 cuft while 200 gallons takes up 26.7361 cuft. I was surprised that the tank slanted side didn't render even more error. For example 1/4 tank on the gauge would actually be 57.5 gallons. I'm pretty sure that topping off my tank from 1/4 gauge level would only take 100 gallons or so (not 150). Thus, the Maletron adapter was recommended. I just don't like the procedure required to program it (empty tank, fill incrementally while programming the adapter). Also assumes my resistance fuel senders are actually in good condition. Headspace above them is tiny. Not sure they could be removed, inspected or replaced.View attachment 114977