Shank to shank is not all that important when applying on 12" spacing provided that the manifold is set up to spread out consecutive shanks so that you do not end up with a bunch of low rate shanks beside each other.  When you start moving shanks out to wider spacings and put higher rates through each shank, it becomes pretty important....you don't really want to have an shank underapplying when you are running 60" shank spacing. 

Yes, agronomy is an imprecise science due to factors beyond management control, however if one takes your position then I guess you may as well just broadcast your corn with an endgate seeded  because you don't know if you want 28,000 or 30,000 population.

We have to do something, so we may as well do the best that we know how.