I am using a glass calibrated tube to measure fuel in each tank showing up to 12 gallons from Sportys that is supposed to be OK for the 150. I have recently purchased a 1975 150M. When I use the tube and it says 8 gallons each side - is that usable or unusable fuel I am looking at. (16 gallons - 3.5 = 12.5 gallons = 12.5 usable) or is it (16 gallon usable)? I am finding conflicting info on the internet and the POH tells me 26 total with 22.5 usable but doesn't answer my question. Thanks
