I agree completely with this test to find when fixer "is shot." My issue is this: if you are trying to keep a low silver level in the final fix tank, say under 1/2 g/l, I doubt this test will show it. (I'm guessing, as I've never done fix-time vs silver content tests, but I'm very doubtful.)The most reliable test is "time to clear". Select a 35mm film type (whether you are using paper or not) and cut an unexposed roll into 2" pieces. Using fresh fix, determine the clearing time. Now, every time you make a run, do the same with another piece of film. When the clearing time is approaching 2x the original time, that fix (first or second "tank, tray or whatever") is shot.