it should vary track to track.
for example, a snare may PEAK at -6 on your sonar meter....
but the RMS value of it could be WAY WAY down....
if you were to use a limiter on your snare going in (assuming you are micing a live drum and use an external limiter) it's feasible you could have those same -6db peaks on the snare hits, but have a much higher RMS value.
RMS and PEAK should be treated as two entirely different things, that are linked together all the same.
if you are heavy handed with drum processing, and want to drive the RMS values of your collective kit UP (meaning, you are getting rid of DYNAMICS)........
YOU SHOULD PROBABLY GO WITH MUCH LOWER PEAK VALUES.
your are doing two things at once....
pushing up, and pushing down.
you have to understand gain staging, and what compression and limiting actually do, to take full advantage of your knowledge about DBFS