On one of my previous 31s, I had a similar fuel fill problem. The cause was a kinked hose coming down from the fill; it had just been installed with too much length. I cut it down to size and no more backflow problem.
112 gallons was the most usable I could pump into each tank. There was a difference between filling on the street and filling in the water because of the attitude of the boat. It seemed to take a few gallons less in the water.