That sounds pretty much exactly what I would do....add 100ppm if needed then change over next time.....pretty safe bet that's what they want this time around. After that...who knows?

Sounds like you are getting pretty dialed in there....going with all RO water is an excellent idea....you know EXACTLY what is going into your rez....not a problem until you have an issue. Believe me I find that one out the hard way myself.

Our well water was about 170 ppms but i could NOT control the ph with it. Yours has 300ppms....kind of scary because what IS all that in there?