Unplug the pump and see how much the sump fills up from the backflow. If it gets close to overflowing, then you have it set up incorrectly.
To find out how much water the system should hold, do what I said above and then mark a line on the sump where the water level is when its done backflowing. Then plug the pump back in and put some more water into the tank.
Keep doing this until you get a nice level where the sump wont overflow if the power goes out. Mark this line and just do topoffs to that level.
Or you can build your own automated topoff system for under $100. You will have to find somewhere to store the water for the topoffs though, and that is why I dont have one.