Thanks Sean and the others -- I suspected that the Phot. Formulary chemicals would last longer than one roll per 500 ml of solution.

I mixed up the Tetenal powders (1 liter) a few weeks ago and processed my third roll yesterday -- the results were fine (the last two rolls were 3 year out-of-date NPH and they optically printed fine on Kodak and Fuji papers).

Frankly, so far my experience with the blix-based Tetenal has been good, so I might forego the use of non-blix chemicals. Kodak certainly doesn't make it easy for amateurs to use their products!