Hi, You would use "Just so you know" if you want to give someone a piece of important information that you think they don't know. For example, "Just so you know, we close at 4 o'clock today"; or, "Just so you know, you should wear a tie for the interview". "Just for you to know" isn't such a common phrase, and it's not clear when you might use it. Perhaps you could use it if you were telling someone a secret; for example, "This is just for you to know. Don't tell anyone else!" It doesn't sound natural as a substitute for "Just so you know".