DIY internal overflow Q's

[ReefDawg]

going to drill the bottom of my 125 for overflows but Ive got a few Questions first...How large should the overflow chambers be? Do you need to access them for cleaning? How long/wide should the overflow 'slits' be? is there a formula to figure out gph flow through the overflow to determine the slit size?

[Clyde]

what I would do would be kinda silly, but easiest

go to a LFS - find a 125G tank, oceanic lest say - find whats the GPH of it - measure its holes - tell em 'I might buy a tank from you'

BUT

I need to measure tank, and see what plumbing I need so I could see if this 125 gallon tank is what I need in my current place and can use my current setup.

that way you can find out how many GHP it is, how large the holes, what it looks like etc.