Sorry, I pressed 'reply' too quickly just now. Foma has very conscientiously listed the Schwarzschild values for each film in the data sheets (unlike – grumble grumble – Ilford, which lists identical Schwarzschild factors for a wide variety of films; honestly, I really can't believe that Delta 3200 and Panf are actually identical in that respect...). Here:
http://www.foma.cz/Upload/foma/prilohy/F_pan_100_en.pdf
http://www.foma.cz/Upload/foma/prilohy/F_pan_200_en.pdf
http://www.foma.cz/Upload/foma/prilohy/F_pan_400_en.pdf
It lists extension factors for 1, 10 and 100 seconds, from which it should be easy to derive a general formula. All films have relatively high Schwarzschild factors; the 100 is quickly overtaken by ADOX/Efke 50, the 400 would be more interesting, as it also has the lowest Schwarzschild factor among the Foma films, at 3 stops for 100 seconds. In my experience, I’d estimate the effect to be a bit lower; feel free to try it out first with some night shots at f/22 before you stick the film into the pinhole camera.
Have fun,
Samuli
