Need help with overflow box

[One_Divided]

I need a bit of advice on the overflow I'm building for my 50.. It's a bottom, corner overflow. I'm just wondering about the slots at the top of the box...

How do I determine where the water level will be in proportion to the slot lengths and how high they come up on the tank? I had the tank built with perimeter bracing, so the highest I could have the water level is at about 17 5/8"..

thanks for any help!

[ 23 November 2001: Message edited by: One_Divided ]