A good way to determine the fixing time of a film in ADOFIX is to stick a bit of unprocessed film (for example a piece of the tip sticking out of your film cartridge) in your mixed ADOFIX (1+4 or 1+9), gently agitate it and stop the time it takes to become clear.
Multiply the time taken by two. This is your recomended fixing time for the chosen dilution.
If you change the film or dilution of your ADOFIX repeat this test!
If you chose the dilution 1+4 your ADOFIX works faster then if you use the dilution 1+9.
The test works also for Photopaper.
You need to test undeveloped material. It may be exposed. The test can be carried out in light.