13 lbs salt is insufficient to regenerate 40K grains capacity. As shown in the chart below, 10 lbs/ft3 (=15 lbs total for 1.5 ft3), is needed to regenerate 39,000 grains usable capacity in 1.5ft3 resin.
For the best balance of salt efficiency, usable capacity & soft water quality, the usual recommended capacity setting for a 1.5ft3 softener is 36,000 grains, as only 12 lbs salt will be needed to regenerate that amount of capacity. This will equal hardness reduction efficiency of 3,000 gr/lb. (36,000 gr / 12 lbs)
The 13 lbs salt that had been programmed, was sufficient to regenerate only about 37,700 grains capacity, but the 40K Capacity setting resulted in greater capacity becoming depleted, resulting in ~2,300 grains capacity remaining depleted after each regen cycle.
The total hardness removal capacity for 1.5ft3 resin is 48,000 grains.
When the softener was initially installed, the 40K capacity setting will have resulted in the 1st regeneration occurring when there was ~8,000 grains capacity remaining within the tank, with that remaining capacity assisting to reduce the amount of hardness leakage passing through the resin bed, even up to the moment that 40K was depleted and regeneration became initiated. Before the 2nd regen, there will have been only ~5,700 grains remaining capacity, then ~3,400 grains, then ~1,100 grains, then 0 grains, each time resulting in hardness leakage increasing.
Even while using the recommended Capacity and salt settings, increased salt efficiency will result in hardness leakage to gradually increase to a specific level in proportion to the salt amount, as shown near the bottom of each salt column below.
Recommend resetting the Capacity setting to 36K, and the salt amount to 12 lbs. Also recommend increasing the softener's Hardness setting 3 gpg higher than your test result. This will anticipate occasions when hardness may exceed the usual test reading, which often occurs when there are multiple water sources feeding into a municipal distribution system.
Define your current Reserve Setting. The softener's Reserve setting, should normally equal the average gallons of soft water consumption for 1-day.
Recommend restoring 100% of the resins capacity. To regenerate all 48K, will require a 1X regeneration with 30 lbs salt. As 10 gallons water within the brine tank would be needed to dissolve 30 lbs salt, instead of a 1X restorative regeneration, suggest performing 2 back-to-back regens as mentioned above, with minimal soft water use between each regen cycle.
Because a single media tank softener cannot supply soft water while regeneration is being performed, any water utilized in the home while each regen is actually underway, will remain unsoftened as flow to faucets will bypass the softener.
Daily softening load: 150 gallons X 37 gpg = 5,550 grains per day
36,000 / 5,550 =6.4 days - 1 day Reserve = 5-6 days estimated regeneration frequency.