I have tried numerous ceiling stones, mostly the cheaper kind, which used to crack on me, and use a 12" BGE right now with exceptional results.
Bottom stones are a 15" cordierite from American Metalcraft and a Fibra-D stone on top of it. All three get nicely hot but I usually stop raising the temp when they reach 850 - 900 F for the bottom and 700 - 750 F for the top, which takes about 15 - 20 mins. The pizza is around 3" from the top stone, but I am able to either lower or raise the top stone, depending on the outcome of the pie and how well it's cooked from the top.
When I raise or lower the top stone in my LBE, it also affects the air flow going over the pie. The closer the stone is to the opening of top vent, the more hot air get forced over the pizza and out the side vent, which benefits oven spring and shortens bake time. The downside, however, is that it takes a little longer...perhaps a minute or so...to get a nicely charred crust going, but in turn, the bottom might cook faster so keep your eyes peeled on how fast the bottom of the pie cooks.
I currently have another idea/modification in the works, which would eliminate one of the two bottom stones altogether and should increase the hot air flow "over and out" immensely. I'll post pics as soon as it's ready to go.
I'd say 2.5" - 3" is best if the temp of the top stone is monitored.
Hope that helps.