This is were you and I differ. You say you use a days worth for a reserve (4*60*16=3600) So 18k-3600=14,400 @ 6 lbs. 14400/15= 960. So 960/60/4= 4 days between regens. 30/4*6= 45 lbs per month And I use 20k @ 8lbs. So 20000/15/60/4=5.5 days between regen. So 30/5.5*8=43.6 lbs per month. So who's math is right? Mine says you use 1.4 lbs more and your's says I use 2.5 lbs more.
You have a serious error in your math; "So 20000/15/60/4=5.5 days between regen.". (the 60 should be 960) but you can't regen on 5.5 days..
You are saying you would regen every 5.5 days but you can't regen half of a day
, whole days only, so it's really every 5 days at 8lbs.
And with 30 days in a month/5 days (you can't regen on half a day) = 6 regens/month * 8lbs
of salt each = 48 lbs/'month.
And 48lbs per month - my 41.25 lbs per month = 6.75 lbs more for you per month.
If I have a Clack WS-1 on a 1.0 cuft and want a regen every 4 days, I use 4 people * 60gals/day/person = 240 * 15 gpg = 3600/day * 4 days between regens = 14400 so I'd program it for 15K and 15,000/3333 salt efficiency = 4.5 lbs * 7.5 4 day periods in a month = 33.75 lbs total while your 5600 is using 48 lbs, for a savings of 14.25 lbs/month for my customer.
If I were selling a softener to that family of 4 in my example above, it would be a 1.5 cuft and regenerate 30K with 9 lbs for 2000 gals on the meter (no calculated reserve required because of the variable reserve in the Clack WS-1 CS) and regen on average every 8.333 or actually every 8 days. 30/8 = 3.75 so 3 regens per month at 9 lbs is 27 lbs compared to my 41.25 in the example 1.0 cuft here, and a very substantial savings in salt of 41,25 - 27 = 14.25 lbs. And IMO salt savings should be calculated on an annual basis and divided by 12 for monthly. There are 46 8 day periods in a year * 9 lbs = 414 lbs per year /12 = 34.5 lbs/month.