Using Fluorite?

I have a 40-gallon aquarium (36x13x20) which will be planted. I bought 2
7kg bags of Fluorite, as this is the amount recommended on the bags.
That's roughly 30 pounds. If I would be using roughly 50-60 pounds of
gravel total, should I count the Fluorite as gravel? I will be using a
substrate heater which I plan to pack in a layer of small-pebble
Conaroc. I suspect that layer itself will be in the neighborhood of 25
pounds. Then the (rinsed) Fluorite, and we're already at 55 pounds. I
guess I shouldn't plan for a very thick layer of gravel on top of the
Fluorite? I had thought to make a "sandwich". Tips appreciated.