for film, you should us a separate fixer bath/s than prints
do a clip test.... take an processed piece of your favorite film
and see how long it takes to clear ( room lights on ) in your freshly mixed fix.
fix your film for 2x that time ( half 1 bath half 2nd bath ) ... when your clear time
becomes 2x the original # your first bath is spent, mix new, and number 2 becomes #1
so your fresh bath is #2 ...

have fun

john