AFAIK, there is no industry standard for determining
capacity. There be no standard then specified capacities
have no meaning. I believe most fixers run at or close to
60%. Some suppliers may be 'cutting'. Trust the numbers.
Of course capacity is dependent upon keeping qualities.
Real world conditions of use are the test of keeping quality.
Now I ask you, have ANY suppliers provided information as
to a fixers keeping qualities under actual use conditions. For
example rapid fixers go bad just sitting unused. And that
includes sealed concentrates. How bad, how fast?
Ammonium and sodium thiosulfate, alone, are odorless.
I'm not in the same rush some are and my volume of
work is low. So, I use the solid, good for ever
concentrate, sodium thiosulfate. Dan
There is no industry standard, that said, Ilford tends to use conservative numbers as quality was always the goal. Some other companies (I don't know which ones) may be less conservative in that economics is the goal.
The best way is to use the clip test, start by always using clear bottles, 1L pop bottles are good for fixer. First look at it, if there is anything floating in it, discard and make fresh.
Put a small amount in a tray or dish, clip the tongue off a 35mm roll, and put one drop of fixer on the emulsion side, wait 30 seconds, you will see a definite spot starting to form, drop in your dish of fixer and time how long it takes for the spot to disappear. This is your clear time. Do this with fresh fixer and write down the clear time on the bottle. Each session you do the test again, when it takes twice as long, discard and make fresh. You can determine your own fixing time, it's at least double the clearing time.
As for shelf life, Ilford says ....
2 years for a full bottle of concentrate...
6 Months for an opened bottle of concentrate...
6 Months for a full bottle of working strength...
1 Month for a partly used bottle of working strength....
I would assume that you can use the same techniques as for developer to keep the bottles full.