I notice that almost everyone specifies stack sizes as a multiple of the big blind. I think in terms of M, the multiple of the initial pot, including antes if any. Isn’t M more accurate as an indication of the relative stakes?

Using M falls into the category of being precise for no reason. Betting starts out in multiples of the big blind, and we can shade things (hands or betting amounts) if the structure is looser or tighter or as a function of how deep the stacks are in terms of Big Blinds!

M is cute and was a good read in Harrington’s book, but it’s not for seasoned players. Paul Magriel was the one who introduced the concept to Harrington, by the way.