What's the absolute max useable fuel in a Duster?

The bottom of the factory tank is large enough in area that I doubt you'd hear any sloshing at all with 1-2 gallons still in it.

It's physically impossible for the fuel pump to access 100% of the fuel in the tank. Even with the pickup touching the bottom, the thickness of the pickup has to be greater than zero. The fuel pump can't draw fuel if there is air present at the pickup. With about 1-2 gallons in the system, air will start to be admitted and pressure to the carb will drop.

The tank is over 30" wide and the flat at the bottom is probably 20" long. With a half inch of fuel down there, that's 1.25 gallon with 1/2" of fuel in there. Plus the lines, which can hold 1/8-1/4 gallon depending on if you have a return line. The filler neck holds maybe .75-1 gallon (assuming 2.5" diameter and 36" long).

If you have a total of 17.125 gallons including lines and the filler, that means you have about 2 gallons left in the tank plus whatever may have evaporated from the bowl and charcoal canister or other vents while running.

Remember, at operating temperature the gas tank is well above the evaporation temp for gasoline - losing 1/2-1 gallon over time wouldn't be unexpected IMO, especially if you don't have a functioning charcoal canister. Non-stop range may be longer as a result, since less fuel should be lost vs when the car spends time parked (in the summer, my vented gas cans for my lawn equipment will lose a gallon in about 2 wks).

With all this in mind, I'd say your usable fuel when empty is right about where it should be for a 16 gallon tank.