well i mix my own dry salt into RO water, and its really easy, i have a 32 gallon brute trashcan on wheels, i dump like 15-20 gallons of water into, drop in a little "30 gallon" heater, and a 300gph pump, and get it stirred up, then i take a measuring cup, and i put in about 1/2 to 3/4 a cup of salt per gallon, let that run and circulate over night, and then the next day, i measure temp and SG, then adjust as needed(i allways end up adding a few more cups of salt) but that way i never get it too salty. I keep mine right at 1.023 according to my swing arm testers... ( i dont have one of thoes fancy things yet)
then after i get the water where i want it, i let it circulate for at least 24 hours, re-test, and if all is well(meaning it matches the tank for both SG and temp), then i remove x gallons from the tank, and then pump this new water into the tank, and i'm done....
now, to keep up with evaporation i have an ATO, all that is, is a device that adds plain RO water to my sump when the water level falls due to evap. mine is on a timer, and it runs 8 times a day, every 3 hours, for 2 min each time...
thats all i do, back before i had the ATO, i would just add plain RO by hand each morning before i left for work and then again when i got home... i was adding about 1/2 a gallon each morning, and then again in the evening, and then entire system volume was about 80 gallons... i tried a couple times, to measure any difference in SG before and after doing it by hand, and i couldnt detect a change with the equipment i have...