Ya I've seen this. It'd be nice to know how high the water "overflows" into the overflow box when they calculate the drainage on that. Cause Im sure a 2.5"x18" constant waterfall is a hell of a lot more than 1200 GPH...

Lets do some math. A 2" drain has a 2500 GPH flow rate according to this site:
http://www.diamondbitsusa.com/index....d=48&Itemid=60

Now, this has a linear surface area of 3.14" (1 squared x pi)

My overflow has a linear surface area of 45" (18"x2.5")...

So I have two 2" drains, so 5000 GPH. Now assuming the drainage through the overflow is the same as through the drain (gravity might help) my overflow technically could handle 35,828 GPH before itd start to overflow the edges of my tank...

This dude has been building tanks for 30 years. He quoted what 1/2" bulkheads and 2" bulkheads are rated at for maximum flow off the top of his head. He had a tank with a 3 foot grand cayman alligator in it in the lobby of his shop.... I'm sure he knows what he's doing.

Lets just say the limiting factor on my drainage is my bulkheads, not my overflow cut out.

Your weir height will be 2.5" below the water height? You only want 1/4" or so of Skimming, the top surface of the water. Thats the whole reason for a overflow. the thinner the "skim" you can take off the top, the better. thats why I went with a Coast to Coast. Using All avail distance for skimming

And you can not take Linear surface and use it like Pipe area. Doesnt work that way.

Alright, so I got them to increase the linear size of the overflow to 22" linear now, so that should be pretty good to do 8x the empty tank volume per hour. That's not a bad amount and I'm sure the overflow can handle more if I need it to.