That will help remove local developer exhaustion from the list of potential causes for your low contrast negatives.
An inaccurate thermometer is another likely cause. As long as your thermometer is consistent, you can (within limits) compensate for potential inaccuracy by increasing development time. If the thermometer is inconsistent it needs to be replaced.
I'm not exactly sure I understand what you are saying with respect to Zone 0 exposure, but if you are wanting to increase the exposure, you need to decrease the Exposure Index ("EI") set on your meter.