Interface = top of the water. Top level = top of the oil sitting on the water. Oil on top = top level − interface. Available water = interface − buffer, at 1.67 bbls per inch per tank. Equalized tanks are connected, but oil sitting on top pushes that tank's water interface DOWN — so gauge every tank and the calculator uses the LOWEST one, since it reaches the buffer first. If tanks are NOT equalized (valved off), figure each tank separately with 1 tank selected.