I'm having a similar problem with my 300 with corner overflows. Apparently, it needs to be 100% level. Not sure if you'll have the same problem with the middle overflow. Only one way to find out
If you're facing the front of the tank, I'd say there's maybe 1/5" more water on the left, than the right, if that. Also, the back is a little lower than the front, but that should be fine since the overflow is in the back.
I woke up at 3am to the sound of running water. I almost shat myself I flew out of bed so quickly. It turns out that I had both ends of a garden hose in the tank, and one end unwound and popped out of the tank and started a back siphon. I only lost about a gallon of water I think. It was still an unpleasant 45 minutes soaking water out of the floor and peeling the carpet back to have a fan blow under it.