My calculation above was based on 8 lbs salt being needed to regenerate 24,000 grains capacity in 1 ft3 of resin. This is a common recommendation to achieve a great balance of capacity, efficiency and water quality.
Because your softener contains only 0.87 ft3 resin, then 24,000 X 0.87 = 20,880 usable grains Capacity.
8 lbs salt X 0.87 = 6.96 lbs.
To dissolve 6.96 lbs, 2.32 gallons water will be needed. With 0.5 BLFC, then the BF setting needed will then be 4.64 minutes. When the BF programming only allow whole numbers, there is little choice but to round-up to the next whole number, ie 5 minutes.
While the hardness removal efficiency for 20,880 / 6.96 lbs = 3,000 grains per lb, the increase in BF time will result in lower efficiency. (5 minutes X 0.5 = 2.5 gallons X 3 lbs = 7.5 lbs salt. 20,880 / 7.5 = 2,784 grains per lb)
Although Reach4's recommendation will achieve lower efficiency at ~2,555 grains/lb, the higher capacity setting will result in fewer regeneration cycles needed per month from your small softener.
The hardness amount in the water should rightly be tested at your location. Often, municipal water is obtained from multiple sources so the usual assumption is for the average hardness from all sources to be programmed. Depending on your home's location in relation to each water source can alter the hardness amount at your location so it's usually advisable to program hardness as 2-3 gpg higher than the test amount to anticipate occasions when hardness may be higher than anticipated. When the actual hardness is higher than programmed, the regenerated capacity will be depleted more rapidly than anticipated.
