ha! you guys, I'm still laughing about the salt on the poison ivy.
In answer to your question, your rainsoft uses something called "proportional brining" which means it keeps track of the amount of water usage, and thus hardness collected, and dissolves the correct amount of salt by adding a specific amount of water. Actually, the last step of the regeneration process is to add just enough water into the salt tank section so as to be ready when it's needed. So that water will be sitting there for days, until it's called for. The computer then checks how much it needs and withdraws that amount. You should never have 20 inches of water in the tank.
As for the salt, ignore the manual. It was probably originally created in 1970, and certain text is just carried over.
Use the salt pellets. Use only a bag or so at a time. You should go through about one 40lb bag per month. Don't fill it up to the top.
Just buy the cheapest salt they carry at home depot, it's all about 99.9 percent pure. You don't need any "iron out" type salts or "solar" salt unless you have a specific iron problem.
Well, gotta go to work.
I'll check the forum later.
anthony