So your 385-400 number is either ppm or mg/l (identical measurement). Since your tests resulted in a hardness range, always use the higher number. 400 / 17.1 = 23.39 grains. Since it is more than 23, round up = 24 grains per gallon.
Using the sizing chart that R4 linked to, for 2 persons, the chart recommends a 1.5 cu/ft system which is 48,000 grains total capacity, assuming no iron or manganese. If your Grand children are expected to stay over often, you may want to obtain a 2 cu/ft (64,000 grain) system instead.
If your water contains iron or manganese, that will skew your requirements even higher. Each ppm of iron is equivalent to an additional 4 grains/gal hardness whereas each ppm of manganese is equivalent to 2 grains/gal hardness. Since it is inefficient for the softener to remove iron especially, it is generally recommended to use other, more efficient iron removal methods prior to softening.
Whatever system you decide, a very high amount of salt would be required to regenerate the entire capacity. Salt efficiency can be drastically raised by programming each regeneration to occur when a portion of the total capacity has been consumed.
The lower section of the chart shows different salt settings and regeneration capacities as well as the expected number of days between regenerations. It is generally considered to be salt and water efficient to regenerate no more than 1X week.
On a 1.5 cu/ft system, using a very efficient salt setting of 9 lbs salt every regeneration (6 lb/cu.ft), 30,000 grains of usable capacity would be provided with an expected regeneration frequency of 10 days for 2 people.
If a Grand child only occasionally stays over, she/he will consume some capacity so instead of 10 days between, a 1.5 cu/ft unit may need to regenerate within 8 or 9 days, but with a metered system, that will occur automatically based on the actual water consumed.
