Robert, be careful. It is easy to get a miss reading with a 'projection' densitometer. There can be spacial and/or temporal variation in the illumination that can throw off your null reading. Spacial variations can be eliminated by not moving the sensor and by moving the negative instead. Temporal variations are probably not an issue unless you are using coldlight. If so, make sure it is warmed up and that you check it at the beginning and end.

Good luck.