Calculating flow rate of PVC

[Pescador]

Are the bulkheads for overflow drains?
Horizontal or vertical?
It also depends on the depth of the hole. (how much water above it)

Here's a chart from R/C
http://reefcentral.com/gallery/showp...p?photo=124520

And a huge thread arguing about flow rates.
http://www.reefcentral.com/forums/sh...s&pagenumber=1

These rates do not apply for applications like returns where flow rates can be higher under pressure.

Try a search for bulkhead flow rates.