Tank Tender

Tank Tender works with pressure/vacuum to be simplistic. As I see it, you have to fully fill at least one tank to know exactly where your marck is at.

Short of that, you could register your known level and then add a specific amount, watch for the increase in the dial and derive it from there.

The second method would not be entirely accurate.

Unless there is a new version of Tank Tender, they don't work on vacuum. They only indicate the pressure at the location of the outlet of the dip tube and display it as inches of water or fuel.

Unless your tanks have parallel sides so that quantity is linear with depth you still need a separate tank table to convert the innage to a quantity unless you plan to have more marks on the gauge than you can keep track of, or use separate gauges for each tank.

It is very simple, if you can pull out the tube that is in the tank and measure how many inches that tube is, you will get and idea of what you have in the tank. If you have a 12" tube in your tank as an example, then you pump up your little tank tender gauge and it read's 6" inches, then you know the fuel level in your tank is reading half way up your sounding tube.
To calibrate it to then know how many gallons you have in the tank to make a cheat sheet or something, you will have to first suck the tank dry and then fill the tank at like 50 or 100 gallons at time (less more or what ever intervals work best) then take a reading of inches and record it to the amount of gallons you had added to tank.

On your Hatteras, the center tanks have a pipe plug on the top of the sending plate. Remove the sending plug on the top of the tank and measure how many inches the tank is deep. I think you can even do this without sticking anything in the tank if I remember correctly. The back of the fwd tank is center bilge all of the way foward in the Engine room, measure from the bilge to the top of the tank and subtract about 1". The front of the aft tank is about a foot or so behind the foward tank......Divide total gallons of tank by inches of heighth of fuel tank, and you should be very close.

OR, if you look on the foward port side engine room bulkhead, there is a placard with the measurements for your vessel on it, and capacities of each tank. I remember seeing it there, or somewhere in the area of the fuel valves.

Shouldn't your boat have mechanical gauges mounted directly on top of your tanks?