Ciao Francesco!
I'll try to help a bit.
The advantage of the Ilford method is that it reduces washing time (the few minutes of fixing time saved is inconsequential).
The idea is to fix for less time in stronger fixer to keep the fixer from soaking all the way into the paper base. Keeping the fixing time to 60 seconds does this.
Then, you can use the wash method described by Ilford here:
https://www.ilfordphoto.com/ilford-optimum-permanance-wash-sequence-fb-papers/
NB: The wash-aid step is required. Without it, the wash will
not be effective. Also, if you plan on toning your prints in selenium, the final wash needs to be minimum 30 minutes. See the information on the page I linked to above.
Now, about the problem of keeping fixing time to exactly 60 seconds. I would try to keep the time as close to 60 seconds as possible. To do this with a large print, you'll likely have to not let the print drain very long.
I would proceed as follows: After the stop, drain the print well. Then immerse it in the fixer quickly and agitate. Pull the print at the 50-second mark and drain it for exactly 10 seconds. Then immerse it quickly in a tray of running water (as Ilford recommends). This will carry over a bit of fixer, but if the water flow is adequate, this will be no problem. Agitate in this tray for five minutes (as per Ilford) and then transfer to the wash aid.
Continue with the sequence as Ilford describes.
You should not have to worry about fixer exhaustion if you fix 10 50x60cm prints or fewer in two liters of rapid fixer diluted to "film strength," which is usually 1+4. If you fix more prints than 10, you will not be fixing them to Ilford's "optimum permanence" standard.
An alternative: If you plan on or need to fix more than 10 prints in a session, you can always refix and wash again later. You can fix up to 40 50x60 prints in two liters of rapid fixer 1+4 to what Ilford calls "commercial standards." This should be followed by the rinse, wash aid and final wash. Dry your prints and take them home. Then, you can refix and wash later in another session to bring them to the "optimum permanence" standard. This is essentially giving two-bath fixation, just split in time. It works just fine as long as the prints are washed adequately in the first session.
I often work this way; I'll do a printing session, make 30-35 prints over a few days, fixing them only once. I wash and dry them and then discard the ones that I don't like. The rest I'll keep for toning and do a toning session later, in which I give another fix, toner rinse, wash aid, final wash and then dry.
Hope this helps,
Doremus