I'd stick with 84 or so. It's cured ich for me in the past.
What do you mean by "how often?" What I'm saying is it's just a set amount -- 2 teaspoons per gallon. You don't continue to add it. You just put it in, however much it is -- mine was a 75, so I put 150 teaspoons -- and that's it. Then the fish sits there in hot, salty water, and the ich die and then you wait another week after you see the last spots on the fish and then it's safe to do a big water change and remove most of the salt.
Like I said, I divided the amount of salt I needed to add by 3, and added a third of the amount each day. But you could, apparently from the sticky, just add it all at once, and then that's it. You don't add anymore. And you don't need a test, because you just add that set amount -- 2 teaspoons per gallon -- and then you stop and wait for the treatment to work.