My oldest daughter once put this in perspective by saying that homeschooled kids always get “A’s” in all of their subjects because if you didn’t know enough about it or got a problem wrong, Mom goes over it or makes you redo it until you get it right. That’s correct thinking in a way, because we do make sure that a concept has been understood well enough before we move on–in most cases.
However, there are times that my kids want to know what a letter grade would be in a particular subject. This seems to be most often true for subjects like math or science (when we use a textbook like Apologia instead of living books), and that is easy to show them because I can grade their tests for Math-U-See or quizzes for Apologia biology, and there is a grade for them to see. We just have to work out whether the grade is averaged or changed if they do poorly, review, then retake the test.