I discovered some time ago that ISO rating of negative film is not based on some medium gray value but on its toe, i.e. on the amount of light where the film starts showing detail. To my dismay I have not been able to connect this information with my light meter readings so far. If I rate film at its box speed and develop it with some standard developer according to instructions: where will the toe be, i.e. how much darker can a region be before it lacks detail? I realize that there are lots of variables involved, some of which can not be held tight in my amateurish setup. I don't expect numbers like 4.765 stops, but would like to get some rough idea.