Paul: There are a number of variables that could have resulted in underexposure. Which film developer was used and at what dilution? With this developer and dilution, is there a film speed loss? If it was processed by a lab, do they use a replenished system? If so, does this result in a film speed loss, and do they keep it correctly replenished? These are just a few of the variables that might effect the final speed rating you use.(posted 8610 days ago)Basing your exposure on the highlights of a scene can be a safe thing to do with slides since the highlights are the thinnest areas of the slide. These thinnest areas have the least lattitude for overexposure, so they need to be handled most accurately. But with negatives, it's the other way around. The shadow areas are the thinnest area of the negative and are the most critical. They have the least lattitude for underexposure, so it is usually best to base the exposure of negative film on these shadows. That is why when using the Zone System, exposure is based on the darker shadow areas that should still show some shadow detail.
You probably would have been more successful if you had metered the shadows and then stopped down a stop or two or three (thereby placing them in the appropriate Zone). It is a little difficult to say exactly how many stops would be required without being able to meter the scene myself. If you are using 35mm, then by all means bracket. If you are working in larger formats where the film is more expensive and processing becomes quite time consuming, I would recommend that you read a few of the books that discuss the Zone System.