Augh! I made the Lemon Meringue Pie from the Pie and Pastry Bible.  I was very careful to follow all the instructions.  However, the pie did not set properly after baking.  Regarding that,I have some questions:
1) Should the pie be on a baking stone at the lowest oven level?

2) Is there any way after cooking it to tell if it will be firm enough before adding the meringue?

3) If the filling is not “set” enough, should I bake it longer? Wouldn’t that ruin the prebaked crust?

I made the Italian Meringue suggested in the recipe.  I have a Cusinart mixer, 1000 watts, 12 speeds.  It seemed to whip up beautifully. I was very careful regarding temperature instructions for the syrup.  When I added the meringue to the pie, it seemed as if I had made too much; it was piled high, really beautiful.  I left it out on the kitchen counter to cool and came back about 3 hours later to see that it had lost almost all of its volume.  It still looked pretty, but there was some minor “weeping”. 

What causes a meringue to fall?  I whipped it for the full amount of time and I was careful with the syrup!

re: the filling, there’s a correction on this site, Rose recommends bringing it to a full boil in a saucepan rather than to 190F in a double boiler. 

re: the Italian meringue, it sounds like you were careful about temperatures, so I’ll ask about quantities.  Did you weigh egg whites (many now are larger than 30g)?  And did you weigh sugar and scrape it all out of the pan so that there was enough to stabilize the meringue?

The filling is supposed to still be hot when you top it with the meringue and then bake it, so that the meringue gets heated/cooked from both the bottom and the top.  Is it possible that your filling cooled too much before topping with meringue and baking? 

re: longer baking, I’m not sure if the oven is the right place to correct a too-thin filling- the crust would be fine, but I’m not sure how long one could bake the meringue for, too much could contribute to beading.

re: baking on a stone, that is primarily to brown and crisp the bottom crust.  You could do that during blind baking of the crust if it seems to need it in order to form a crisp, golden brown crust, but I don’t think it’s a good idea for the second baking of the filled and topped crust, as it might contribute to underbaking of the meringue.