To some extent, the use of commas is an art rather than a science – a matter of the writer’s personal style and intended meaning.
However, following these three patterns of comma usage will help make your sentences clear:
• Use commas to separate items in a series.
• Use a comma in a compound sentence before the coordinating conjunction (and, but, or, for, yet, so) that joins the two independent clauses (complete sentences).
• Use a comma to set off nonessential elements that precede, interrupt, or follow the independent clause.
My personal use of commas is all about the way I would speak. If I have a short pause when I say/read a sentence, I use a comma to show that pause. Certainly not always right, and I do follow the rules you’ve shown, well,,,,, I sometimes over-commatize!
Perhaps “Professor” McGrath will finally learn that punctuation is our friend.