My parents moved into a home last year in Las Vegas area that had a water softener. I believe the water here is extremely hard, about 30 gpg if I'm testing correctly (basic aquarium GH test kit), and coming out soft around 1-2 gpg at most.

I'm trying to make sure the Fleck 5600 Timer is set correctly for them. They are a two-person household, but they mostly shower at their rec center after classes, so fairly frugal water use, say more like 1-1.5 person equivalent.

1. The resin tank is 8x44. How can I tell how much resin is inside? Is it most likely 0.75 cu ft?
2. I've read here that the optimal interval for regen is 7-8 days. However, if I understand this 12-day Timer correctly, you can only set even intervals at once per 12 days, or 6 days, 4, 3, 2, or 1, correct? There's no way to set 7 or 8 days?
3. The salt dose is set to 6# on the back of the Fleck. I understand the grain capacity of a given cu ft of resin depends on the salt dose. I think only the 6 day or 12 day cycle is appropriate for my parents' situation. If I find out how much resin there is, should I then adjust the salt dose up or down to more closely match the grains of hardness they're actually using?

The reason for the third question is to be accurate and frugal on salt usage. Less for the cost, but moreso that my parents are elderly and can't carry the 40# bags of salt. So if I can refill the brine tank (14" square x 36" high, I recall it held about 3 40# bags) only once or twice a year on visits, I'll save them the hassle of having to refill.

Lastly, now that I understand water softeners, I'm thinking about getting one for my own house - 3 persons x 13 gpg hardness. Already familiar with the Fleck 5600 so may go that route.

4. If I want an optimal regen interval of 7-8 days, then should I go for the 5600 metered, as the timer version can't do it?

2 & 3. At two people and 30 gpg you need about 3600 grains removed per day. 3/4 cube of resin @ 6 lbs of salt (actually about right) = about 18k capacity so regeneration should be every 5 days so you have to choose 4 days and be safe or 6 days and chance hardness leaking through when the resin is exhausted. I'd choose 4 days.

4. Fleck 5600SXT for you.

If you fill the brine tank all the way up you invite bridging and mushing. Only keep enough salt in the brine tank to cover the water and visit your parents more often... they'll like to see you more often.

Timer softeners are old technology and not near as efficient as on demand initiated softener control valves.

With 18 gpg set the timer @ six days with 6 pounds of salt.

TDS 430 ppm
Hardness 18 gpg

Will set timer as you suggest.