Nr Fairy: You won't get equal flow from each tank. As it's gravity fed the levels in the tanks will drop together (like a manometer), but the main having a larger surface area than the aux will provide more fuel. I haven't done the math, because calculus was a long time ago for me. 2/3 main 1/3 aux works as a guesstimate. Or two hours / one hour and use the guages like a clock.