How you delineate your AA is the most important CRA decision you will ever make because it affects not only your results, it affects the "performance context" which in turn drives performance standards. In the 25 years I have been advising banks about CRA I've seen too many banks take the easy way out and annex whole counties when they have only one branch in the county. This makes it difficult to serve the entire AA because it is too large and all the LMI tracts that are outside your practical assessment area will inflate performance expectations.
What we do is use "live mapping" to advise banks about the various issues and tract combinations in different AA configurations to determine what is the best AA delineation for a bank.
CRA Exam Preparation, CRA Performance Evaluations, Key Performance Benchmarks, & maps