I have recently been running some tests in which I develop a single sheet of film at a time in my SP-445. I generally use D-23 1:1, which, conveniently enough, has a stated capacity of 4 sheets per 16 oz of D-23 1:1 (though it's not exhausted by then; some use it significantly more). As inexpensive as D-23 is, I would still prefer to use those 16 oz. more than once in such tests, if I'm doing them in one session.

The question, therefore, is, by what percentage do I increase the time for the second and third runs? (I don't mind mixing fresh for the fourth.) I used to reuse Microphen (a much more energetic developer, and I was using it stock-strength) for 9-10 rolls, increasing development 10% per roll developed, but that's my only experience with developer reuse.