Well the last photo I took had pretty decent exposure at 15 seconds. After exposure, I metered the scene with my Program Plus on Auto. It selected 1/60s at f/16, with 200 film.
I have a pretty good knowledge that my camera is f/300 because I measured the pinhole with my enlarger. Difference between f/16 and f/300 is four stops slower lens=less light.
Now the difference between 1/60 and 15 is about 10 stops slower=more light. The difference is 6 stops.
6 stops slower than 200 is ISO 3. Who knows how accurate this is due to reciprocity, but it's something to start with.