County yields can be found in the Quickstats section of the website for the National Agricultural Statistics Service: http://quickstats.nass.usda.gov/
Since a flex lease give you part of the risk of production, you and your tenants will be involved in the decision as to which farm program to choose for the farm's base acres acreage. Your decisions must be unanimous for each FSA farm.
When CRP acreage is released by FSA or exits the CRP contract, the base acres that existed when it was enrolled in the CRP contract return as they were and program yields will be reassigned as they were prior to entry into the CRP program.
The owner of the land owns the base acres and payment yields associated with the land on that FSA farm.
All programs make payments on a percentage of the base acres on the farm, no program makes payments on the actual planted acres. ARC-IC calculations are based on actual and historical farm yields. The actual planted acres on the farm in the crop year are used to weight the revenue numbers in the calculation. In that way, actual yields and planted acres do impact the program and payment calculations for ARC-IC. By comparison, ARC-CO uses actual county average yields and PLC uses the program or payment yields to calculate payments.