"Slightly off topic, but how do you use a step wedge in testing a negative? I have used mine for paper, but don't know to accurately expose film through it."

The procedure I use is based on the BTZS testing system developed by Phil Davis for his Zone System workshops and described in the last several editions of his book Beyond the Zone System.

Basically you expose five or six negatives to a step wedge, develop them, and then read and plot the film densities. Each negative gets the same exposure but a different time of development, ranging from about 1/4 to twice or three times of what would be considered normal development. I use 4X5" Stouffer step wedges for film testing.

One of the keys to this system to the ability to accurately expose film consistently. Many people use their enlarger and timer for exposure and that works fine if you can expose accurately at about 1/10 to 1/5 of a second. Exposures longer than 0.5 seconds should be avoided because of reciprocity. However, if one really wishes to make any meaningful comparison of relative speed between films you must have a more accurate exposure system than most darkroom timers allow. For my tests I use a Metrolux light integrator/timer that provides accuracy to about 1/100 of a second.