RE: usable fuel
The clunk should be just short of the back of the tank and be able to get close to the corners in both inverted and upright flight. You'll never get every drop of fuel out of the tank, but if yours is quitting with 1/4 (10%?) tank left, you may have a pin hole in your fuel line at that level and as long as the line is submerged in fuel does not affect it, but soon as you burn up enough fuel, the hole is exposed and viola--she conks out. Flatspin is right--dual pickups, one is bound to suck a little air in a dive/climb when the level gets low. Put a new piece of fuel tubing on and make sure your clunk flops around just short of the back of the tank.