overflow height help please

[eh1421]

what determines water level in a tank using a overflow
i am in the process of building my 180 gallon wall tank and i am planning on putting to 1 inch overflows one on each end
now how far down should i place them?
i just want to make sure the water level will be above the 2 inch border around my tank.

was thinking about using these
http://www.drsfostersmith.com/produ...&Ntx=mode+matchallpartial&Np=1&pc=1&N=0&Nty=1

what do you all think

help please mike

 

[repair]

The placement of the bottom of the hole determins the water level. The water level is normally about the middle of the hole.

You can call if you need a better explination.