Thank you all, guys. I will try 50 and 25 ISO.
Itīs almost sure Iīll get foggy background so I will try to use it in my favor. Which kind of subjects you think the fogged background would help? Portraits? Night shots?
On the other hand If I shoot in a really sunny day with high constrasts from light to shadow areas would the foggy background be less noticed?
I had a go at some Ilford HP5 (not the plus version), about the same age as your Tri-X. I shot it at 200 ISO, but used the normal 400 ISO development times (d76), and the result turned out fine.
Good question, Henrik.
What about developing? Should I consider as a normal 25 or 50 ISO film? Or should I develop as 400 ISO?
After doing exposing a roll at all the possible EI's you might want to try, at normal EI 400 time and temp, and decide which EI gives you the shadow point you want, then shoot a roll at that EI and cut it into 3 or 4 sections for developing.
The test sections can then be developed one at a time adjusting development to find the best for you.