I think it’s safe to assume most of us have taken one of those Mary Sue litmus tests at some point. I also think it’s safe to assume most of us have certain qualms with a question or two or three or seventy on those tests. My major qualm is how they usually vilify self-inserts. Yes, most of them have disclaimers saying the tests are only guidelines, and I honestly wish more people would read those before mutilating their characters in order to get scores in the single digits or negatives.
Self-inserts, if done right, are usually better than non-self-inserts. “Done right” simply means that you insert the parts of yourself that will help you write a more convincing character. If you’re a programmer, for example, and you’re writing a story about an indie game company, I see no problem in making your protagonist a programmer. If you’re a historian and you decide to make your protagonist a historian as well, you can write a more convincing character than a general writer who’s only ever imagined what it’s like to be a historian.
Adding bits of yourself into your characters adds legitimacy to them. You can go too far, which is why those Mary Sue tests exist in the first place. That’s when you have a character who’s essentially you, plus (mostly) everything you ever wanted to be. I trust you can figure out when you slip into that area, though.
Writing prompt: take your career, job, hobby, passion, defining physical trait, socioeconomic status, etc. and write a character with one or more of those. See how much more honest your writing seems.
The Poets and the Peddlers 