I use reef crystals. It works very well for me. I use a 44 gallon brute to store rodi, and another to mix saltwater in, and two other ones. LOL
I do water changes for a living sooooo...... I mix it up and use quickly. I use a 5 gallon size plastic drill operated mixer for paint to mix my water. I make it as soon as I get there, mix it, and then leave it to mix with a powerhead while I clean the tank. In the winter I use a very large aluminum pot, to boil rodi water and add it back to the salt to heat it. I just kinda know how much to use.
Some customers do thier own partials so I leave them mixed saltwater in 5 gallon gas cans. They just remove and replace, and then every other water change I come and do it. Seems to make them happy. LOL