I think the only way to know if it will not overflow is to just try it. Mine will overflow if the pump runs continuously, so I have a timer installed. I had the same problem finding the right size tube. I used a piece of pvc that was a little too big and just reduced the outside diameter of it with sandpaper until it fit snugly. They also make a rubber grommet to fit between your drain hole and the tube for a snug fit, but you end up with a smaller diameter tube.