I think we are really talking about the same range: 3 gal/a in each of two twin rows on 30" centers is about the same as 6 gal/a on single 30" rows isn't it?

On straight single 30" rows it seems like just 3 gal/a isn't worth the trouble to plumb the planter....but I could be wrong. I'm the engineer and steel guy, not an agronomist.

Bill Moyer could probably offer better fertilizer advice than I can. I just report what I have seen work for various customers. 

Jim at Dawn