Chet,
There are a number of criteria that go into determining the watershed for any given area...it can be taken to the level of a somewhat complicated mathematical equation. I have said this before, and I will say it again...Pub 590 contains all the basic formats for the elements of this equation. If you care to take this calculated route, you will need to know the hydrological types of the soils that make up this watershed and a snapshot of the method of crop planting within. This, combined with your avg precip in your area and some other basic criteria will provide the required calculation elements. If your NRCS office is "pond-friendly", you will be doing yourself justice to get their input and opinions(this forum has reported various levels of competance and interest by the NRCS...hope your's is pro). From the hip, I gotta guess that you get a good level of precip. Assuming this, even with medium to highly permeable watershed soils, 120 acres of farmland water shed will be plenty to feed a 4 ac pond. 30:1 is strong.
FWIW, I think Meadowlark in TX has a 1 ac "sky" pond with virtually no drainage and he says it does fine (hard to believe, eh?...ML is a straight shooter).
(edit a Pub 590 link)
Pub 590 
