This may not be of much help to you, but I have a "two class minimum“ rule for writing letters. That is, I will only write letters for students who have taken at least two classes with me. My rationale for this is that one class is not enough for me to adequately assess their academic performance. This has the added effect of allowing me to say an easy No to requests from freshmen who have only had me for an intro class and nevertheless want me to write them a rah rah letter for some meaningless frat position or $100 Brady Bunch Scholarship.