Andy, the quick answer (that I can recall) to distinguishing between an underdeveloped versus an underexposed negative is to look at the edge markings. If they are gray instead of black then the negative is underdeveloped.(posted 8317 days ago)If you've already established EI 64 with APX 100 by calibrating your shutter, meter, and developer then please ignore the rest of this response. If you haven't then you'll have to find your EI first before looking for your normal development time. The conventional measure for the right EI given your equipment and procedures is 0.1 density units above the film base + fog.
Here's a link to an old (two part) article by Tom Halfhill on personalizing your film speed. If you prefer the calibration method mentioned in Adams' "The Negative" then you might find this Zone System site interesting. The best hint I read for testing film is to use a diffuser.