What I did to determine the fuel capacity of my 65 7ECA (nominally 39 gal) was to fly until I had just a small amount of fuel (~10 gal) remaining, then tied down the tail (blocked up to normal flight attitude), and drained almost all the fuel. I then got in, started the engine, and ran it until it quit (fuel starvation). I then filled the tanks, 2gallons per wing at a time, while also marking a fuel dipstick. My 39-gal tanks took 39 gallons to fill. So I guess the unusable fuel (6lbs, per the TCDS) means that the tanks are really 40 gallon tanks, but labeled with their “useful capacity” instead...
By the way. I also calibrated my dipstick with the tail down in “taxi” position... The dipstick doesn’t show anything until there are 10-11 gallons in each wing... So when I fly with my instructor, I fill to the dipsticks lowest reading with the tail on the ground for a total of 20 gallons. Up I mostly use the dipstick as a “sanity check” for the fuel flow meter I spinstalled. It is deadly accurate in terms of fuel burn, but you still have to watch for leaks and /or fuel theft if you’re ever tied down outside... (Ask me how I know...)