MAG Flake alone is OK, but also using Epsom salt may be a bit better, especially for routine use. It helps prevent chloride from rising relative to sulfate in the tank water. I just use MAG flake to boost my IO, then once in a great while I use Epsom salts by itself.
FWIW, I'm not a fan of the Seachem product as it is not ionically balanced. Kent, ESV, and the DIY are all better balanced.