This wont work properly. The baffle after the foam block before the return pump should go higher. This last baffle will determine the height of the water in the sump.
Factors involved in determining the height:
-How much water capacity you want to the sump to have left over in case of a power outage
-How deep your skimmer should be submerged for optimum efficiency. (Although you can always make a shelf for the skimmer to sit on to adjust its height. Ive stacked eggcrates to do this)
-If you want to add a refugium section or place for live rock.
I would use a wall that is 2-3 inches shorter then the first wall.
After this wall, the level of the water in the return pump chamber will change depending on how much water is in the whole system. It will slowly drop due to evaporation. This is a good spot to put a float valve if you have a rodi unit or other top off device. With that setup, its another chore you wont have to deal with for evaporation is replenished automatically.
Theres many ways to do this, but if you want to see mine, it can be seen here..I chose to place all my pumps external to keep the heat out of the tank.
http://www.tonmo.com/community/index.php?threads/7888/