Your amp settings should not matter as the room fills up. That should be a function of the sound man correcting for variations in the room. One thing that I do is use my amp basically as a monitor. I set my amp on a stand, pointing back at me off to one side and away from the audience. This has two effects. For one, it allows me to keep the volume of my amp down, while allowing me to hear the actual sound of my amp that my Microphone is hearing. With the amp positioned behind me, the volume needs to be higher for me to hear it, and it always sounds like there are less highs than there actually are. The result is that your guitar sound through the mains is different from what you are experiencing on stage. You also push out your guitar sound from the stage to the audience from your amp vs. through the PA. With the amp positioned so that it points at you, and away from the crowd, you get a better mix through the mains, which is where you really want your band's sound to come from. The ideal setup is to have all of the sound anyone hears from you to be coming from the mains. If you are properly set up, and the mix is coming from the mains, how full the room is will not matter on your amp settings, and will be adjusted by the sound man. The mic on your amp isn't on the other side of the room, it's about 2 inches away from your speaker, so the sound from the amp speaker to the mic won't be changed by how many people are in the room.