You want the probe to be in pretty much the hottest place in the enclosure. From your description, it sounds like you have it in the right place. If you are using additional sources of heat other than the heat lamp, then you might need to relocate your probe or have more than one.

However, if "reflextics" means the stuff I am thinking of (sort of like bubble wrap made out of aluminum foil), then I don't know why you would have that inside your enclosure on top of the substrate? So I may not be understanding what you are saying.