Works great. I've been making and using it for a few yerars.
You can buy Arm & Hammer baking soda anywhere.
If you only occasionally need to supplement, you can use QuikJoe IceMelt, Prestone Drivewat Heat, Peladow or DowFlake for calcium. If you need to dose everyday, you should get the calcium from www.bulkreefsupply.com
If you only need to slightly boost mag, you can use EpsonSalt. If you need to supplement mag regularly, you should use EpsonSalt mixed with magnesium chloride hexahydrate from bulk reef supply.
Just follow the article for mixing and use the reef chemistry calculator to figure out how much to use.