If your meter is compared to another, and they don't agree. You don't know which one is wrong. Put a third meter into the mix and it may be closer to one or the other.
Now you have three variables. And 4,5, etc. eventually you may find one that matches.
If you process your own film, You just introduced several more variables into the mix.
Start with ISO, shutter accuracy(30% tolerance),aperture,water quality,temperature,developer,agitation. anything else?
You're determining a standard for YOUR particular situation. If the meter is off it doesn't matter as long as it's consistant.
Want a real test? jShoot a roll of film &* compare it to an older negative that you processed.

Green grass, Gray card, clear blue sky. Pick one.