Marco, I tried to pursue such a prediction tool at the initial stages of my venue into photo chemistry: it failed miserably. Poorly buffered solutions are extremely sensitive to minor environmental changes, and pH will be all over the place. Many solutions will also change pH over time, because aerial Carbon Dioxide would dissolve and lower pH, because some compounds oxidize and thereby change pH, or because somewhat volatile parts go away over time.
If you don't believe me: try to find a tool which correctly predicts pH of 1% Sodium Sulfite. There exists a range of online pH calculators, and since you have access to a pH meter, you can easily check which pH estimate works (hint: none will). None of the tools I found back then account for ionic strength phenomena, and the formulas which would account for ionic strength would rely on many constants which I was unable to find in published literature.
Your best bet is sticking to
well established buffers if you want to reach some pH without a meter. Once you add extra stuff to these buffers (like other compounds found in photographic recipes), all bets are off.