I don't get why the mix would be louder when you turn down the monitors. If you are using proper recording levels and leave the master at unity, the mix should end up at about the right level. If you want to hear everything louder, turn up the monitors, if you want to hear it quieter, turn them down.
Mix level and volume are two different things governed by different controls. You really need to get a handle on not compensating for one by adjusting the other.
As was mentioned, you need to monitor at different levels to help get perspective on how things are sitting in the mix. The 80-85db thing is just because that is where our ears have the flattest frequency response. It has nothing to do with ending up at the correct mixing level, it's more of a tone thing.