It doesn't mater how short the bag is, if the tank is an inch below the cap with air, the engine will quit, This is a constant pressure system to the fuel pump as long as the tank stays full you won't have air in the system... 45 degrees in any direction is the best way to use the tank, but I've seen them laid flat, up-right and on there side and still work perfect... As soon as your main tank is empty, you are done flying... the tank won't help...

Bruce