How much do you want to water your plants? I just mix it and not over think it, sometimes not enough if I'm rushin, but I don't have to water as often when I don't put enough in. But too much and water runs right on through...just my 1 cent. Srry, not sure about size. I know 6" usually suffices.
When you say river sand, is it bc it's so coarse like fill sand and not fine like brick sand or play sand or recovered beach sand ( available in the gulf coast areas)
Good point about the salt , I would not use fine sand bc of compaction, well maybe a sprinkling ( I've been guilty of that) but not anymore, and the untreated river sand here is contaminated with sewage runoff. Nasty lol