Yes, this is very inaccurate. Your overflow is automatically downregulated by your return pump and the water it can push back up to the display. You need to make sure your overflow is rated for at least what your return pump is returning but preferably higher.
For example, you can have an over flow rated for 1000 gph and a return rated for 1gph at your head pressure. You will effectively be circling 1 gph through your sump.
If on the other hand you have an overflow rated at 1gph and a return pump rated at 1000gph at your goven head pressure. You will effectovely be overflowing 999gph over the rim of your display and onto the floor as your overflow is only able to bring 1gph back to your sump. With this scenario, your ato will pump into your sump until your ato is empty which will drop your salinity. You will eventually have an equilibrium reached after your pump is running pretty much dry and is returning the 1gph back into your display as it is drained through the overflow.