On mine, I fly it balanced just aft of the wing tube. I have also flown it balanced just ahead of the tube, and the aft position is much better. Balance it with an empty tank. 20 oz tank is much bigger than makes sense for a 26cc engine. Your engine will consume approximately .88 oz/minute at full throttle. 20oz tank will provide 22.75 minutes full throttle, you will be hauling unneccessary cargo most of the time for your duration flight, and you will see a great shift in CG as the 20oz of fuel burns off. Unless you are doing some sort of marathon with an aerobatic plane I can't think of any reason to be carrying that much fuel. A 12 oz tank will give you around 13.5 minutes full throttle run. If you do not fly full throttle all the time, the actual flight time will be much greater.
There is so much BS regarding the balance point on this model. It's not that complcated.
It is a really great flying plane, I'm sure you will enjoy it. Have fun!