Lol, you still dont grasp the concept. Maybe I can help.
1" main line, 10 people in the house, 90 grains hardness, customer wants to maintain decent efficiency and regenerate approximatley every 6-7 days at 6 pounds of salt per cu. ft.
10 people x 60 GPD x 90 gpg x 6 days between regeenrations =324,000 grains system needed for this application. So you would have no problem installing a 15+ cu. ft. single tank system? According to the logic of Gary, a 30" diameter tank will have no problem with residential flows. The recommended folw rates are just that, recommended. They are design parameters that need to be considered. They are "recommendations", not mandates. Why is it a bad idea to recommend a more appropriate design for an application? A 4 cu. ft. system should work ok. Is it ideal? Is it the best choice? No, but it may be slighly cheaper than a proper twin alternating system.
Stop looking and digging for conflict where it does not exist.