Wally, use what is the norm and you can't go wrong... or become a softener engineer.
mialynett.... My experience with most national brand dealers and their sales force is that they are like shyster lawyers and used car salesmen. It's all about the money and they over charge for undersized equipment.
If you don't want to size for the peak demand of the household's water use, don't, but don't say it shouldn't be done when all resin manufacturers and their distributors publish data so delaers can size correctly.
BTW, I've already explained how your 1.0 is not allowing breakthrough yet.
And yes, I know you know it all but for one thing, you don't know what you don't know.
If you want to find undersized equipment, ask your prospective customer, service and new equipment types both, if they can say their present softener has always given them 0 grains/gal soft water and see if they don't tell you "no, there are times we get hard water". Then see if you can figure out why that is if not due to exceeding the constant SFR of their softener.
As to the 6lbs for a 1.0 cuft... man it's adjustable down as far as the control valve you are using will allow you to go; salt efficiency doesn't stop at 6 lbs/cuft.