The position of your stone is going to vary from oven to oven, it all depends on what works best for you. With my oven for example, the heating element is on the top of the oven, and unless I have the stone on the middle rack, the top doesn't brown quick enough. I preheat for an hour, so the stone is plenty hot. It takes some experimentation.
I have tried blowing a bubble underneath, but the air doesn't seem to trap well. I will probably practice getting the bubble underneath in conjunction with flour, and as I improve my bubble technique, use less and less flour. I had read in here on another post that you should put your stone in the lowest position in the oven, and I guess that had blinded me to moving the stone up a rack position or two. Only one out of the three pizzas I made Saturday was blackened enough underneath that it added a bad flavor, so all was not bad!