I use a 16oz tank on all my 50cc planes. (DA50 in Yak 86", WH Edge 28%, and AirWild Extra 260) I fly 13 minute flights (timer) and have between 1/3 and 1/2 tank left. I've flown 20minutes before but I was very nervous. Mostly I fly 3D. Mostly slow flights 1/2 to 3/4 throttle. I'd recommend a 16 oz tank.
My QQ weighs 16lbs 4ozs. Mine was very tail heavy also. I added 6ozs of weight to the front the first few flights. After about 5 flights I started taking an oz off every flight. You guessed it...after 6 or 7 flights I had removed all the weight and was very satisfied with the flight performance. You do have to be a bit careful on landing but no problems really. I think I'm at about 178MM right now. After I got used to it I'm fine with it there. I love this plane.
Thanks
Barry