I was reading your posts and thought I might add some thoughts. If you get a new tipi, remember that if the canvas wasn't pre-shrunk, you'll get a decent amount of shrinkage and an 18 could be a 17 really quick. Also, I have a 16, and although I like it, I will definitely go with an 18 or 20 for my next one. A 16 is great for two to three people, but any more than that and it gets a little crowded. ( I know that there are people who say that you can cram more in there, but hey if you're going to use a tipi you might as well be comfortable!)
As to the question of the leaning and the placement of the fire, I don't claim to be an expert, but I prefer the fire to be a hair closer to the front of the tipi than the exact center. In my tipi, I don't like the fire being right under where the poles cross, so I move it up a bit. I think this is only because its a smaller tipi if it was an 18 or larger, the fire probably wouldn't be situated underneath the poles. (I don't know if any of this is actually true because I can't look down at my tipi's set up, it only appears this way.)
one final thought, the tipi rings you were talking about earlier might not be an accurate reference to the size of the tipi. When it was time to take the tipi down, the rocks that were sitting on the liner were simply rolled off (like pulling a table cloth out from under the china) so the rocks were rolled slightly in from where they were actually placed.
I don't know if any of this helps, but oh well
Scouter