overflow questions

[00nothing]

I jsut picked up a 125 gallon and in some weird twist it had a vertical overflow in one corner but the tank was never drilled so i cut the overflow out and am going to put it on its side and cap the ends to create more of a calfo style overflow.

Here is my question what should my minimum dimensions be for approximatly 1000-1500gph the overflow box will have beans overflow system for drainage

I know there is some weird calculations for flow and all that but honestlyt i am not that smart

and am hoping someone can tell me something like 4 inches out from the glas and 18 inches long or something