Doremus is right, but I'll give you a quick idea.(posted 9083 days ago)Part 1: to calibrate your film processing =to calibrate your ISO=to calibrate for the low-Zone densities.
Part 2: to calibrate your film processing also= to calibrate for density/contrast=to calibrate development time/temp/dilution as per any given original luminence (sort of 'pulling' or 'pushing' or 'normal').
Part 2 is more obvious, but Part 1 is just as obvious if you read between the lines: perform (at least mentally for now) the following test:
take a number of exposures of a flat area on the same roll of film at a Zone 1 density (i.e., set your exposure to 4 stops under what the meter reads); if the recommended ISO of your film is, say, 400ISO, then I would suggest you expose the frames in the following order: 1) ISO 400, 2) ISO 320, 3) ISO 250, 4) ISO 200; 5) ISO 500, 6) ISO 640, 7) ISO 800 (I always screw the middle numbers up, but suffice is to say just keep exposing at 1/3 ISO increments plus-and-minus at least one full stop, i.e., with ISO 400 film, go plus-and-minus in 1/3 increments at least from ISO 200 to ISO 800...
...once you have exposed the whole series on one roll, then develop the film normally. The first frame with a density change (in general), will correspond to the proper ISO for that film, as used in conjunction with the same chemistry to develop it. I.e., if the first notable frame occurs at the ISO 320 setting, then with that developer and film combination you should always rate that film at ISO 320, EVEN IF the manufacturer states a different ISO. This can often lead to a big difference (many photographers rate TX at 200, not 400, for e.g.).
Hope this helps, but I still go with the following poster: get The Negative by A. Adams
shawn