I want to really venture into the saltwater side

[Sillyfishies]

A R/O machine is just used to make your tap water usable for your tank. You can use it to mix with salt or as top off water to replenish lost water due to evaporation and skimming. I make about 10 gallons a week for a 55 gallon tank. 5 for water change and 5 for top off.

Summed up pretty good. A RODI unit just takes almost everything out of water. You then mix it with salt to make the saltwater. When water evaporates all the salt stays in the remaining water. So you top the tank back off with pure RODI water.

The "gravity measure" you want to get is a refractometer.