To get a more accurate description, you really need to look at the peak flow from a given inch rain. A 300 acre flat, sandy watershed will flow much less runoff than a steep, predominantly clay 300 acre watershed. The NRCS guy is most likely correct though. 300 plus acres will likely be a lot of water in an 8 inch rainfall event.
My advice is to look for a smaller watershed to build on. Generally, a 300 acre drain will have a smaller tributary drain coming into it from the side that you could safely construct a dam across.
