Yes, this is excellent.

Keep the answer simple and direct. Say exactly what you mean, and don't let them pull you into justifying your boundary. Whatever you say is the final word on the matter.

Speaking as someone who was very codependent, and therefore very "other" focused, the toughest part for me in regard to boundaries, was learning to use the word 'I' to start/state healthy boundaries. Much of my life, if I started a sentence with I, me, my, etc, I'd be accused of being selfish or narcissistic (of course by people who actually exhibit those traits).

When it comes to good boundaries, it's important to speak from only your own space. No talking in terms of the other person's experience or feelings. No starting sentences with "you" or "your", or even focusing attention on anything that person might need or want. There's a codependency quote that says something along the lines of, "You cannot set a boundary and tend to the other person's feelings simultaneously." It's important to accept that reality. They have to deal with the consequences and their own feelings.

And never forget that "No." is a complete sentence.