Please don't apologise you are not being impertinent, you are asking a perfectly logical question. I use the manufacturers suggested ISO and control the density of the negative by development which has been determined by testing. My reasons for placing the shadow on Zone VI and not Zone III relate to a growing concern over a number of years of using Zone III. I grew increasingly unhappy with the lack of detail in the shadows in my negatives and decided to give them an extra stop of exposure which resulted in an immediate improvement. Clearly I had to modify my development times but that was relatively simple. I have always felt that to over expose and underdevelop would produce a better and more printable negative. Incidentally, at the time that I went through this trauma I spent some time with Bruce Barnbaum when he visited Scotland (I live on the English side of the Border) and he told me that he had come to the same conclusion about shadow placement.
Please don't think that I am trying to sell my current book in this fine forum, I happen to think that commercials are not really acceptable, but do have a look at it for I do comprehensively cover film exposure and development in the first chapter.
I hope that this answers your question, if not please get back to me.
Thank you for the interest.