I think fixing silver bromide in excess sodium thiosulfate may reasonably be assumed to be a pseudo first order reaction. The concentration of unfixed silver at time (t) would = initial concentration of silver times e to the power of (-k times concentration of thiosulfate times t). So half the concentration of fixer means double the fixing time. Well, close enough, anyway.
Do you have any data to back this up? From what I know, fixation is anything but a first order reaction, see here, especially pages 3 and 4.
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