Pretty simple, really.  Running with the rows you take out 2 rows out of every 64.  Running across you take out  22.4" every 960", at least with my tires. 

Anyone that thinks they are going to drive "between" 15" rows is as nutty as the guy who thinks he can tell how much air is in a truck tire with a hammer.   I can't do it reliably with 11.2's and it's just impossible with 14.9's.