it's impossible to 'balance' any sort of direct return without having an air tight sump or a computer controlling the water levels.
the way you've described your setup, it definately sounds like you are trying to do a direct siphon from the tank to the sump.
the return to the sump though, it is either I have to much or not enough.
sorry, but this method will not work and is doomed to failure. even if you get things balanced so they look to be equal, you will end up with a mess and possibly a burned out pump.
to get water out of your tank you need an overflow, which externally is two boxes connected with a siphon tube, internally it is a piece of plastic surrounding a drain hole/pipe.
hope this helps