As long as you’re hearing the same perceived volume, I guess it’s all that matters, right? Did you take the sound difference blind test? I can’t detect any difference below 1dB, for instance. At least for that test tone. So that should establish your own (ear/human) tolerance.
Procedure-wise, I just play this noise track while pointing the decibel meter as close to the source as possible – i.e., at maximum level within the ear cups.
Volume-wise, it works great for me when comparing several cans in the same listening session. See this post for an example in action:
Cheers.