Save yourself alot on nozzles and shutoffs, but the narrower your spacings the better coverage per plant you will get. Spraying a 30" row crop, with 20" or 30"
spacing, is going to put more nozzles directly over the row than 15" spacings where one is always between the row. Same on 15" rows and 15" nozzle spacing.
10" is ideal for coverage. Its not rocket science. Like I said why dont you go to 40" or 60" or even 120" spacing? Let me know how it turns out. We are talking 
COVERAGE not gpa's. If he wants more coverage narrow up the spacing. Yes, 20 gpa is 20 gpa regardless of nozzles, but not the same coverage as nozzle
spacing. I'm tired now. :)