incase others end up here doing the same thing, here is my attempt to break the formula.
0.01 = convert the whole number percentage into a decimal percentage
500 = My tank 500 size in gallons
35.65 = ft3 per gallon of propane
the formula below produces “Cubic Feet of Propane in Tank”
state: "{{ ( states('sensor.tank_utility_xxxxxxxxxxx') | float * 0.01 * 500 * 35.65 ) }}"
I think this is correct and it should be easy for other to use this formula on their setup.
edit: I might have the the ratio of ft3 to gallons wrong. https://dec.alaska.gov/media/18793/gasconversionchartpublic.pdf shows that the ratio should be 35.65ft/gallon.
edit: The above equation takes into account of errors that I had with the 35.65