Do you know if manufacturers rate fixer at exhaustion, or just where results will fall off and quality will decline? I have had no issues using Kodak fixers to the rated capacity, and I find that even my 'dead' fixer will still clear a clip of film in about 4 minutes. I know at some point results will taper off, but I kind of go by the double clearing time... kind of. 5 minutes continuous agitation, or 3x clearing time with continuous agitation, whichever is longer. I have no issues with underfixed film, even with the last roll I run through before dumping into my big jug.