Sound level meter settings

My sound level meter can be set to slow/fast reading, A or C weighting, can be set to hold maximum value and has two measurement ranges of low at 30-100dB and high at 60-130dB. There seems to be a discrepancy of around 10dB average between the reading speeds (with slow displaying the lower readings), as well as a difference of a few decibels between the A/C standards (with C displaying higher values). What combination of settings should I use to determine sensible listening levels? At what levels do you guys listen for extended periods?

Most of what I've read recently recommends the C weighting for music as it has the wider frequency spectrum. My meter is identical in spec to this and I've also rigged it with a similar disc to go over the cans -

I'm now getting better readings, with the discrepancy between slow/fast response due to previously not giving the slow setting enough time to average out the measurements. I find C weighting, slow response and high range to give the most balanced measurements, but it's led to the discovery that my normal listening level is perhaps not sensible for extended sessions at around 86dB average. I've turned my dial down to limit the average to 80dB but I find the HD650 looses some detail and is more congested at this level. That value still seems high for marathon sessions though.

80dB with C weighted measurements are equivalent to about 72dB readings on the A setting with my meter. This aligns with levels I've seen people post in other threads. I didn't really want to have to turn my amp down any lower, so if the A measurement is the one I should be referencing, 72dB seems like a sensible level.