I use fresh fixer for my film because that is the one thing I can't redo. I also use fresh fixer for my 2nd fix bath when archival printing. I use used fixer for my 1st bath or when only fixing proof or work prints. I use the exating scince of 'guesstimation' to determine when the fix is exhausted. My methods may not be the best, but I don't like cutting corners when it comes to long term things (negatives, hanging prints).