Somewhere in the forum archives there is a thread about the actual tank volumes for water and diesel... Because the tank is V-shaped at the bottom (where the float-gauge is installed), '1/4 tank' on the gauge means only a couple of gallons of usable diesel left. 1/2 full showing is more like a quarter tank in reality. I'm sticking by 38 gallons...
On my old H37C, I emptied the tank, cut a clean-out hatch near the gauge, cleaned the interior and refilled. As I added successive 5 gal jerry jugs of diesel, I noted the reading on the gauge and made up a table so I could accurately estimate the fuel remaining from any gauge reading. Unfortunately the table went with the boat when she was lost.
On another note, I found it was really easy to 'burp' a bit of fuel out the air vent on the stern. On my boat, the vent line was routed from near the top of the tank, down into the bilge and under the shelf that holds the hot water heater, and back up to the vent on the transom, thus trapping a bit of diesel at the bottom if I over-filled or if I sailed on port tack with a full tank. The solution is to re-route the vent line so that it leads "straight uphill" from the tank to the vent.