Hello all. I apologize in advance for posting yet another water softener sizing thread, but I have a dilemma I can't quite wrap my mind around. It mainly stems from the fact that both the SFR and the softening capacity are both proportional to the volume of resin. Here is the problem:

White scale is ruining our glassware, and it is all over the stainless steel dishwasher and plumbing fixtures. Moreover, we have a Rinnai tankless water heater (9.8gpm) that we are very happy with, but I suspect is very susceptible to this stuff.

Despite our symptoms, by labwork, our water doesn't seem that terrible;
Municipal supply; Hardness ~ 10 grains per gallon; ph = 7.3; TDS 153 ppm, iron 0.

We have two adults and three kids (ages 6,4, and 2), so by conventional calculations, we should use a minimum of 300 gallons per day. Checking my water bills in the last 18 months, I only use about 130 gallons per day -- perhaps because my kids are so young and we have high efficiency appliances. So, even at the higher usage estimate, using a 7-9 day regeneration cycle, I need only about 24,000 grains of capacity.

On the other hand, we have a 4 full-bathrooms and 2 half-bathrooms. The master shower has an overhead showerhead and two body jet showerheads. We are considering adding a third powder room and a jacuzzi tub in the distant future. (About 31-34 wsfu.) According to the International Plumbing Code, with the number of fixtures in my house, I should have flow downstream from the softener of 25 gallons per minute. (Which probably isn't realistic anyway since I only have about 50psi on a 3/4 inch supply. I think it's one inch into the house, but then the main line into the mechanical closet is smaller.)

It would take maybe 4 cuft of resin to acheive that flow rate. But I would have to set my salt dose so low, it would be a huge waste of water. With 1.5 cuft of resin, it seems like I would have more than enough capacity, but not nearly enough flow. If I go with a compromise of 2.5cuft of resin, I am concerned that I would have both insufficient flow, and also way too much capacity and wasted water.

All this, and I would prefer a system flexible enough to handle the changing the water usage of my family as the kids grow up.

As a compromise, I am zeroing in on a 48000 grain system with 1.5 cuft resin -- ResinTech CG-8 looks like it may allow flow as high as 10gpm/cuft; the rest are all considerably less. Salt dose of 4-6. Regen time override at 9-10 days, and a turbulator to try to minimize channelling. I'm thinking I might have to change to a bigger resin tank if I have issues with bleed-through as the kids get older.

Finally, I am thinking of a Fleck 5000 SXT since it can theoretically handle up to 21gpm which I may need someday, but I don't read a lot about prople using it. Is there any reason why? Should I just go with a Fleck 5600 SXT instead? I'm a little scared by the flow limitation compared to the 5000.

Sorry for the long thread, but hopefully the details will help. If anyone can offer a better choice of resin, valve, or overall system design, it would be greatly appreciated.