Matt,
I'm in a similar situation to yours, read the books, mapped topos, etc. It seems to me your watershed is quite a bit larger than you estimated based on the map above. I think it actually extends from the NE corner of the pond along the ridgeline to the northwest to a point where the picture is cropped off, then back down to the southwest where it intersects your property line. If the watershed you have outlined is 30 acres I would guess you have roughly 15-20 acres more outside your propertyline. Like I said I'm not experienced at this but based on my research this is my opinion. I'd be interested to hear from someone with more experience to confirm or refute.