IngMacca,
Your test is not adequate to determine if the paper has been fully fixed. Unfixed compounds in the paper will have a degrading effect much later...
And, despite what many say, the clip test isn't really adequate for paper, especially if you are processing fiber-base paper and trying to process for optimum permanence. The acceptable dissolved silver level in fixer for film is much higher than for paper processed for optimum permanence. The silver test strips also only start to indicate after a single fixing bath has reached or gone past its minimum level of dissolved silver for optimum permanence. They would, however be useful for two-bath fixing regimes, for film fixer and for processing to less than optimum permanence.
To answer your question then: There is really no "simple" and accurate way of determining fixer capacity for paper, particularly when processing for permanence. The whole issue of adequate fixing is more complicated, but worth learning about.
There are, however, a couple of methodologies that will ensure good fixing for paper without going to a lot of trouble.
The first is simply to keep track of the number of prints you put through your fixer and then follow the manufacturer's recommendations for throughput.
Unfortunately, there are a couple of different standards out there that confuse the issue. The less-stringent method often called "general purpose" or "for commercial use" or something similar, and assumes a less-than-maximum print lifespan. In other words, prints processed "for commercial use" are not processed for optimum permanence; that's a different standard. The capacity numbers given are often for "commercial" standards, not "archival" ones. We need to be sure that the capacity numbers we use are for the degree of permanence we desire.
I assume, since you are in Italy, that you are using Ilford or Tetenal products. I recommend highly that you get the Ilford tech sheet on Rapid Fixer and read it carefully, especially the sections on two-bath fixing and silver concentration. The document is here:
http://www.ilfordphoto.com/Webfiles/2006130218312091.pdf
Note that Ilford's "commercial standard" is 40 8x10 prints per liter, but that that drops to 10 8x10 prints per liter for "optimum permanence." Here's the quote from the top of page 4, second column in the section on silver concentration:
"For prints that need maximum stability for long term storage a the maximum silver level in
the fixer should not rise above 0.5 g/l i.e., approximately 10 20.3 x 25.4cm (8 x 10in) prints [per liter]."
If you fix using a single fixing bath, and wish your prints to last as long as possible, then stick to the 10 8x10s per liter capacity. Or, alternately, you can opt for two-bath fixing (which I use and recommend). With this method, you can use the first bath up to the 40 8x10s per liter (of bath one) since the relatively fresh second bath completes the fixing process. The process is described in the Ilford document.
If you wish to take the next step, it involves getting into process control a bit. The idea is to test your fixed and washed prints for residual silver (adequate fixing) and residual hypo (adequate washing). There are a couple of easy-to-make or find test solutions for this: ST-1 (Kodak's Silver Test 1) or selenium toner for determining safe levels of residual silver and HT-2 (Kodak's Hypo Test 2) for determining if the fixer has been adequately washed from the print. Using them is not all that complicated, but this post is long enough already. If you're interested, search here for more posts from me and others on the subject.
Best,
Doremus