Glenn...
Always a challenge...specially on one nighters in different size rooms.
In very large rooms, I turn off my monitor (which runs directly out Monitor Ouput), put on a midi, walk the room and tweak as necessary setting it on the low side for the first part of the job.
Then, adjust my monitor.
As the room fills up and I find the monitor is more difficult to hear, I gradually increase the main volume to compensate for the crowd noise. I pay special attention to the tables nearest me making sure it is not uncomfortable for them to speak without yelling at each other.
If I am off base, someone will usually come up and ask me to turn up or down.
Smaller rooms are much easier. There I can often position speakers behind me without feedback so I hear what they hear, knowing it will be a tad louder for me than them.
And in very small situations (house parties, rest homes, etc) I use a powered speaker just off to my side, tilted up at 45 degrees. In this situation, it is very easy to tell what is comfortable for everyone.
Eddie