With regards to the size of tank, I'm getting 9:30 per flight at 2/3ish throttle and still having about 1,7 litres after landing, time doesn't start till the gear goes up.
I weighted (CG) my Rebel with about 1/4 tank , 1.5 liters fuel and full Map tank (6 oz) this gives me some ballast forward and I'm at 15mm ahead of the rear tube.
Netural is at the back of the wing tube something to consider when assembling.