This will not answer your question but maybe will give some more insight.
In my case of a 3 acre pond, my watershed area was not enough (as estimated by my NRCS agent), so he designed a terrace in our nearby farm field that pulled more water into the pond. This put us at what he considered barely enough watershed. I need to actually measure it, but I am guessing about 10 acres watershed for a 3 acre pond with a 42" annual rainfall.
At times during the summer during a hot dry spell we can get down to 18" below full pool. I don't like the pond that low and the lack of flow through causes the water to get pretty yucky (technical pond term). If we have a wet spring or an extra rainy season we can be running over the emergency spillway during a 12" rain in a 48 hour period.
In dry years without large rain events I wish we had more watershed to get some additional flow through the pond to remove excess nutrients (from fish poo from feeding). Then when we have saturated soils and a large rain I'm glad I don't have more. Getting the perfect watershed size for a pond is difficult and it is different for a drought year than it is for a wet year. So a person has to try and hit a happy medium. As Bob stated, your NRCS shoukd have the experience for your area to be of help.
Some areas large rain events are unusual, so will need relatively larger watershed area. Large rain events are somewhat common for us here in SE Ks, as the link to pictures show of our pond.
what our 3 acre pond looks like after a large rain event 
