I would suggest having two cans, as you said: one for salted water and the other fresh. This way if you have a mishap or want to do a water change just for the heck of it, you are ready to go, and don't have to wait for the R.O. to make 20-30 gallons of water.
As far as the ratio of water changes, you are using oceanic salt as stated- notoriously low on alk
. (I don't ever get a reading of 8 btw) So even if your in tank alk
is used up and at a dangerous level, and you do a 50% water change, it's not going to return it to 8.
Get a salifert alkalinity kit. they are easy to use and a lot more accurate than API. Test every couple days for a couple weeks and then you'll know how your alk