How close to the top should I fill a pie to avoid overflow? I'm planning a pot-pie recipe in a casserole dish and was wondering how high I should fill it without worrying about it overflowing during cooking.  A traditional vented pie crust will completely cover the contents.

I will put a baking sheet underneath to catch overflow just in case, but I'd like to have some general guidelines.
A basic bechemel or broth based pot-pie filling will not expand significantly when being baked.  It may bubble a little under the pastry from being simmered.

You would the casserole up to maybe 1/2 inch (1 cm) below the lip, and apply pastry on top of that.

Less is fine if you have less filling.