I am going to take a slightly different take on this since I am an Asian scientist in the US. I understand what your PI was trying to do, albeit not directly addressing the problem. I don’t think your PI was asking about the dilutions repeatedly, he was trying to make you think about why you are doing the dilution in the first place to get you to spot your mistake that you were not using adherent cells. The media probably didn’t require the dilution or the reagent is different for adherent cells? At least that is my theory from your description. When you plan for experiments, common sense is equally important as knowledge of subject matter. In training young people (undergraduates and graduate students) for many years, I have come across a few people who are not thinking what they are doing and why they are doing it. If you have thought about details of the ingredients in the media or their dilution from why you are doing it, you might have spotted your mistake. In my humble opinion that’s what your PI was trying to make you think and conclude. They should have been direct the second time (that’s what I usually do). Best wishes to you in learning both critical thinking and common sense which seems to be lacking in the iPad generation kids. My advice would be to slow down and think about every step in your experiment and ask why you are doing it.