Interesting musings.
I think a good measure would be word length, in k (thousands). For example, a novella (by SFWoA standards) is between 7.5k and 17.5k words, novella between 17.5k and 40k, and novel greater than 40k. And anything less than 1k is a scribble.
Progress might be referenced (a la Kindle) as a percentage. So, p50 would be halfway through, regardless of length of the prose. p1 would be the first page (or very close to it--maybe p0 is the title page?) and p100 would be the end. You lose some precision wrt p-books that have more than 100 pages (and gain some for shorter works) unless you add a decimal. Of course, this is a "relative" measure, as opposed to an "absolute" page number, but only relative measures like this will survive reformatting/reflowing.
More musings...