What's the feel on variable rating (prescription) for beans in midwest (NE IL for me)?

If one has the capabilities, should populations be based on prior years yield data? 

Would you normally INcrease population in poorer areas (sandy knolls, etc) and DEcrease it for the highest yielding areas?

What if you create the prescription on April 20 but don't plant until May 10? Some suggest increase bean pop by 1000 every day after May 1. Not sure about that...just considering advice.

Or....just leave it alone & plant beans from one end to the other?

Thanks