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
My suggestion of leaving it ten minutes is so there is no doubt that whatever fix you are using, and however exhausted it is, the film 'should' have cleared by then. I was not suggesting that 10 minutes fixing time should be considered normal or that 10 minutes should become 20 minutes.