Fredericka, I'm not sure the baking stone does much for pre-baked
pizza. It's purpose is normally to drive heat up into a raw dough. The
direct contact with the stone transfers heat into the dough faster than the
air contact in the oven so the loaf cooks (at least initially) from the
bottom up, providing a strong oven rise. For most pre-baked pizza there is
little for the oven to do except heat the pizza and provide a little final
browning for the crust.
For baking bread, I turn the oven on with about 35 minutes lead time. It
takes about 20 minutes for my oven to get up to temperature so that leaves
an extra 15 minutes for the stone to get uniformly heated. My baking stone
is heated by the electric element which is directly below the stone. In
effect it is the stone which heats the oven. If your stone is far above
the heating element it may take longer for it to be uniformly heated.
You may also find that your baking procedure and times will change a bit
with the stone. I found that it was best to start with a high temperature,
475 to 500 F, then turn the oven down to 400 or 425 F immediately after
putting the loaves in the oven. (Lower temperature for larger loaves,
higher temperature for baguettes, rolls.) The hot stone drives the heat up
through the loaves while the dough is still soft enough to allow the bread
to rise, then the air in the oven slowly bakes the upper crust. You'll
know it's right when the bottom crust is the same color as the top crust
and the crumb gets to the right temperature at the same time the color is
right, 195 F (moist crumb) to 205 F (drier crumb).
Werner