When you are calculating the "correct exposure" ("Correct exposure" being the amount of light necessary to achieve a negative with a given density) you are actually playing with shutter speed, aperture and film sensitivity. If you restrict those variables to a fixed value, the only things you can do is overdeveloping or underdeveloping the film after the shooting or 'guessing' (Or calculating) what light you need to get a correct exposure and then shoot only in those situations.

If you are in the second situation, I recommend you the f/16 rule. (for any given ISO sensitivity under direct, not very harsh sunlight, you need to shoot at the nearest speed to that ISO and f/16. e.g. ISO400, 1/500. f16). Or, if you have a light meter handy you can measure with those values and see if a given spot or area is within the acceptable lighting range for your settings.

If you actually want to calculate the actual 'amount of light' needed you will need the specific curves of the film and previous tests with the film, developer and fixer you are going to use, which takes time and (to be totally honest) is a royal pain in the ass. If you are curious the Ansel Adams trilogy has everything you need to do this (Book 2 talks about this procedure throughout the chapters but doesn't have one specifically for it), if you are looking for a practical approach to take photos without much hassle it will not be worth it.

If, as I guess, you are doing this to calculate the correct exposure on a pinhole camera, you should know that pinhole cameras are essentially cameras with fixed aperture. You can caulculate the aperture (f number) simply by dividing the distance from the pinhole to the film by the diameter of the pinhole. There are several other useful formulae you can use, like d=sqrt(2fw) [f:f number; w: wavelenght of the light you want to see sharper] for the actual pinhole diameter. You have plenty of resources for this online, and following them will be way easier than going to the brute-number approach.