Lena- You crack me up. Thank you!

Az- You keep repeating 'how do you know that the readings on your SPL meter are accurate?' and going on about variables. It doesn't matter if your meter isn't reading an exactly correct SPL when you start. The meter could be off more than 10% percent in either direction without becoming useless. (I can't believe that I have to explain this, but I will anyway.) I'll say it again. You are looking for a reference point in your mains to which you will balance your surrounds and center. Get it? Your meter could say 'x', and if you set your other speakers to 'x' your levels would be balanced. And all of the variables that you mention are relevant to varying degrees, but don't overcomplicate things. The meters have omnidirectional mics so pointing them at the ceiling while sitting in your listening position is a good compromise. As far as other listening positions, you are going to have those very same variables whether you use a meter or not.
The meter is a tool. It gives you physical representation of what you are just 'guessing at' by not using it. The variables remain the same with or without the meter. The thinking man uses all tools at this disposal.
Do what you will, but can we move on to something else now? See you on the next topic.
Mix