You get a maximum K of capacity of 45K out of a 1.5 and 90K with a 3.0 cuft (both) @ 15 lbs/ft, not 48 and 96K.
The 1.5' has a constant SFR of 12 gpm, the 3' has 20 gpm.
You'll have to ask AK why he says the 3.0 needs 20+ gpm to work like it should; there is no such need. It means you can run 20 gpm through it and not suffer hard water bleed through. Exceed the SFR of the cu ft volume of resin and you get hard water through all softeners.
If I were still in business I would suggest a Turbulator (can't use gravel) and no top basket (Turbulators have their own) and 3.0 ft of SST-60 (using a 1/3 cup of Iron Out every 4-6 weeks in 2 gals of water and a manual regeneration) with a valve that an handle a 3.0 cuft softener.
The side tube on a Turbulator should be set 1-3" below the top of the resin bed (at rest) and usually comes preset that way for the tank you are using if you order the softener with a Turbulator.