CB,
No expert here, but I can give some practical advice as I also constructed most of my pond this time around and have learned from mistakes made along the way.
You need as much freeboard above your overflow pipe as possible. Freeboard is the space from the top of your overflow pipe to the top of your dam. I would go 2 feet minimum, and more if you've got the dirt. Your emergency overflow should not come into play until the water is at least to the top of your pipe, perhaps a bit above assuming 2-3 feet of freeboard. That said, make sure to keep in mind any structures or buildings that could be affected by raising your emergency overflow level. I'm in the flatlands of Illinois, and a few feet increase in pond level could put the pond at your back door. Sometimes lowering your overflow pipe, thus pond level is the better option. It was in my case.
My original contractor thought that 1 foot was enough. $10,000.00 and a whole lot of my time spent on a dozer says he was wrong. Spend the time and money now to do it right and you can enjoy those 2-3 inch rain events inside listening to the rain patter on the roof instead of outside watching the waves lap up against the top of your dam. I've been there. Not fun.
