[QUOTE=250swb;1466326. Put a spot of fix on an end of roll film leader and leave it for ten minutes, the spot will clear a patch on the film. Then drop the whole leader into a cup of fix and time how long it takes for the rest of the film to match the cleared spot. That time is doubled and that is your fixing time,

It may be that your choice of 10 mins for clearing time was merely to illustrate the twice clearing time principle but in fact if it takes 10 mins to clear and therefore 20 mins minimum fixing time then I think users should be seriously questioning their fixer's efficacy. At these times the silver content in the fix has, I believe become saturated and should be dumped. Using 250mls of correctly diluted fix for 135 films I find that the silver test tells me that the fixer is exhausted after 5 films so I dump after 4 to be on the safe side