Take a trip to sunny south Florida -- should be about 82 + F today. Seriously take a house plant outside if you want leaves or some objects that would appear to be in the same zones also you could photograph tree bark. Use your gray card a vary your exposures. another way to find your personal film speed that would not require even going outside is: set an 18% gray card in a constant light, meter off of it, with the lens cap on click off a couple of frames, don't focus but fill the frame with the gray card and take +2, +1 1/2. 1, actual reading from the gray card, -1, -1 1/2, -2, then develop the film. When you are ready to print: take one of the unexposed frames and make a test print to find how much exposure from the enlarger it takes that will show a slight change from pure black, cut pieces of printing paper print each negative at the time you just determined, (label each piece) the one that is closest to neutral gray is your film speed. For example if the +1 (ISO400) rate at 200. Depending on a particular situation your shooting exposure or development time can be tweaked but this is a good starting point for standardizing.