Very good explanation there. I don't worry all that much about hardness breakthrough for a big tub all that much because most of the time they fill that will soaps and scented oil and other herbal crap that is powerful enough to keep the water hardness from being much of a problem. Of course if someone else is showering at the same time they may be a little pissed off but again, I'm betting that situation won't come up often enough to be a problem. I also would reduce through the softener to 1-1/4
You are missing the importance of having used capacity that is not able to be regenerated with the salt dose being used, and how that affects the water quality until that resin is regenerated fully. That has to be done manually, no control valve/softener can do it automatically.
Gary Slusser Retired (= out of business)
Click Here to learn how to correctly size or program a water softener. CAUTION, as of Nov 12 2013 all YouTube videos showing how to rebuild a Clack valve have an error in them that can cause damage.