It really doesn't matter who makes the test solution. By its very nature the test is not that accurate. Part of the reason is that it requires the user to make a rather subjective decision. PF in their directions should give the recommended capacity for the fixer. As fixer nears the end of its useful life it becomes harder and harder to remove silver complexes. This can cause degradation of prints and films which may not be immediately visible. Film and paper are expensive, fixer is cheap. Scrimping on fixer is a foolish economy.