John, as far as the bottom of a pizza is concerned, it makes no difference where the stone is in the oven. You turn the oven on, the thermostat will hit a certain temp, shut off the burner, then the temp will drop a bit, the burner will turn back on- over an over again, creating this 75ish deg. window. Eventually the heat will conduct all the way through the stone/plate. The stone will reach this temp where ever it is in the oven. It will reach this temp faster on a bottom shelf, but it won't get any hotter. When you launch the pizza, the heat that's stored in the stone bakes the bottom of the pie. During the bake the bottom burner is useless- the pizza will be done baking by the time the heat from the bottom element makes it through the stone. So, during the bake, you either use the broiler, or nothing at all, never the bottom.
The stone position is critical to browning/leoparding on the top of the pie. If you got a steel plate that can do 2 minute bakes, you need to position the stone close enough to the broiler so the top bakes in the same amount of time. For Neapolitan, 2 minutes, this means very close to the broiler- top shelf. For NY 4 minute-ish bakes, you might be able to get away with the next slot down.