Look for real numbers to be released by NASS about March 1st. RMA uses the NASS county estimate. In 2008 NASS adjusted their release dates so that values would be released before insurance sign up deadline. Here's the NASS website when their county yield estimates are released: http://www.nass.usda.gov/QuickStats/Create_County_All.jsp
You can use some sort of district estimates but that has a lot of noise in that data. Especially if you are working Southern Michigan. To get some idea of potential claims the RMA indemnity maps will give some hint.
http://www.rma.usda.gov/data/indemnity/2008/120108map.pdf This is a December 1st picture and should represent the "yield" losses. Later pictures will be of the "revenue" losses which will distort the yield message.
To get a back of the dinner napkin estimate: On the revenue side corn is 76.5% and soybeans are 77.5% of spring price. So just an average yield will pay 80%, 85 and 90% GRIP policies. | 
GRIP estimate
