what is hardness efficiency
This is equal to salt efficiency as specified in post #27. Hardness Reduction Efficiency = Regenerated Capacity divided by the pounds of salt needed to regenerate that capacity.
Ex: Because 12 lbs salt will be needed to regenerate 40,000 grains capacity in a 2 ft3 softener, then 40,000 / 12 = 3,333 grains per pound hardness reduction efficiency.
Hardness leakage is when the softener resin does not remove 100% of hardness and so some hardness passes through the softener. The ppm leakage amounts specified in the chart, will not occur immediately upon using those salt settings, but will build-up to those amounts after numerous regeneration cycles. As hardness leakage increases, the quality of the softened water decreases.
Although grains per gallon is the common hardness measurement when working with a softener, ppm or mg/L are smaller values usually measured and specified by testing labs. 17.1 ppm or 17.1 mg/L = 1 grain per gallon. While water with 10 ppm hardness will be soft, it will be slightly harder than water with 6 ppm hardness.
Water treatment typically involves compromise. An 8 lb/ft3 salt setting will usually offer the most acceptable balance of salt efficiency, capacity and water quality. When a user wants even higher salt efficiency, then 6 lbs/ft3 is usually recommended, but that will come with the penalty of lower quality water and more frequent regeneration due to a lower Capacity setting. Even lower salt settings will further increase efficiency, but regeneration will be more frequent and the water quality may be lower than desired.
For your family to use an 8 lb/ft3 setting instead of 6 lbs/ft3, will further increase your 2 ft3 softener's available capacity beyond your family usage within 30 days. While the water quality will be higher, salt efficiency will be much lower than usual because a greater amount of unused capacity will be wasted (re-regenerated) every 30 days. A smaller capacity softener would have been better suited for your hardness level and water usage.
Depending on the amount of unused capacity remaining at 30 days, it may be worthwhile to not worry about wasted capacity so your family has use of the water quality provided by your chosen initial 6 lb/ft3 setting.