Note that fixing has more variables than just the amount of chemistry that's in the bath: agitation, paper type, emulsion type, ambient temperature, freshness of fixer, ... getting reproducible numbers might be quite a challenge unless one creates very reproducible start conditions. This could also mean that in some instances we could fix much beyond the (most likely conservative) numbers given by Ilford.
If you want to do reproducible tests I'd say you prepare AgCl and AgBr (from AgNO3 + KCl/KBr + filtration + drying) and season the fixer with predetermined amounts of AgCl/AgBr. You don't need to prepare much seasoned fixer as you can test fixation and washing with small test clips. This test setup allows to to quickly determine how much Silver can remain in fixer for archival fixing with various fixing and washing techniques.
Once you have these numbers, all you have to do is translate these back to your practical printing setup.