Most resin manufacturers say their regular mesh resin will treat up to 5 gpm per cuft. That is a bit on the conservative side.
As to efficiency... normally that is figured as you would figure fuel efficiency in a vehicle; fill the tank, drive X miles, refill and then divide the miles driven by the gallons used. You don't use remaining gallons or any type of evaporation rate....
Salt efficiency is the regenerated K of capacity (30K per cuft) divided by the lbs of salt used at a max of 15 lbs per cuft or less. 30,000 divided by 15 lbs gets a 2000 grains/lb salt efficiency. So the fewer lbs used the higher the salt efficiency.
I.E. if you use 15 lbs/cuft and have a 3 cuft softener the total is 45 lbs and that gets you 90,000 grains of regenerated capacity at 2000 grains per lb of salt.
Same 3.0 cuft softener 'short salted' would be say 6 lbs/cuft for a total of 18lbs/regeneration and that would get you 20,000 grains per cuft per regeneration or 60,000 total for a 3.0 cuft and 60,000/18 lbs. shows a 3333 grains/lb salt efficiency.
That's a 1333 grain per lb increase in salt efficiency, or some 30+% increase.
But... you get more frequent regenerations at 18lbs each over whatever time frame you want to use UNLESS you buy a larger (cuft) softener that allows the 90,000 capacity with 6lbs per cuft. Which gets you the 90K at 3333 grains per lb and the same number of regenerations as the 3.0 would get you. And, you can usually program to use damned near if not the same number of gallons of water per regeneration as would be used in the smaller softener.
Most residential softeners sold today are metered, meaning they use a water meter to trigger a regeneration. That's called a demand regenerated softener and most will be delay regeneration meaning the regeneration will be started at the industry standard 2:00AM although most can start at any time of day or night but, you want the regeneration when water is not being used in the house/building or you get hard water past the softener until the regeneration is finished.
So... metered/demand based softeners regenerate when a set number of gallons of hard water is run through the unit.
You get the number of gallons by multiplying the total gallons used per day (7 * 60 = 420) by the number of days you want between regenerations (8) = 3360 (gallons on the meter).
Then 3360 times the compensated hardness (27) = 90,720 rounded up to the next 1000 = 91,000. Then 91,000 divided by say 20,000 (that's 6 lbs/cuft or a salt efficiency of 3333 per lb/cuft) = 4.55 cuft softener, and 6 lbs times 4.5cuft = 27 lbs. per regeneration or, 91,000/3333 = 27.3 lbs round to 23.5 if the control valve allows it, otherwise go to 28 lbs.
But... there's always a BUT...
If the control valve doesn't have the variable reserve feature, you must subtract a day's worth of capacity (or gallons) for the reserve so... 91,000 - 12,000 (one day) = 79,000 divided by 420 gals/day = 2940 gallons on the meter.
So I suggest 7 * 60 = 420 * 27gpg = 11,340 grains per day, times 8 days = 90,720 round to 91,000 (91K) and that divided by a salt efficiency of 3333 = 27.3 lbs of salt per regeneration or round to 27.5 and if the valve won't allow that, then 28 lbs in a 4.5 cuft softener.