If you have not added salt yet, I would pull some water out in a new rubber 29 gallon trash can and mix it out side the tank, then pour it back in. Do you have any power heads running in the tank yet?
Tank is 3/4 full with R/O water. Can I mx the salt in a jug with R/O water and dump it in to fill the rest of the tank? I have read to let the salt sit a while in the water prior to adding it. Does that only matter after tank is up and running? I have two powerheads and two filetrs running.
You really need to let it sit once you have fish in there to stablize the new salt water and not stress/kill the fish. But no need if you are just starting to cycle the tank because you need to run it for a month or so before you start stocking the tank. Mix it and add it ( assuming there are no fish in the tank yet!)
Mix it and pour it in. Run a few power heads in the tank to keep it mixing and you should be fine. Again, the "aged water" is for water changes in up and running systems.