Use a 2 bath method, it takes more fixer to start, but ultimately, you will still consume the same amount. Exhaustion schemes have the problem of averaging - 120 black prints will exhaust far less than 120 white prints, so they have to average. You can get a testing solution (lifetime supply for a few bucks - I have a couple of small Edwal bottles from the 70s). Test the first solution till it starts to show failure, then, as Ian says - dump the #1, move the #2 up to #1, and make new #2. You'll never have underfixing.
You can find plenty of opinions on this by searching this forum for "2 bath fixing".