Not sure of any formula for the size of overflow box other than on RC's (here) where they mention linear weir length.

Picture is of Skimmin's set up. (clearer picture here). Those boxes were handling ~1000gph each.

http://i193.photobucket.com/albums/z...t/IMG_0832.jpg

Ya, I'm not sure on the overflow box size either. I haven't got that far in my plans. :lol:

I don't think you'll be able to regulate the flow well enough on your drains without putting a separate gate valve on each one. Now, I'm just wondering here, but is there a reason you aren't drilling your returns?

I've decided to make them 17cm long, 20cm deep, and 10cm wide.

I'm going to do that now too. See my next diagram. (Yes, sorry, another one, LOL.)

I'm not drilling the returns because I'm not sure if the sump will be big enough to take up the access tank water during a power failure.

Gotcha! ;)