How Much Perlite Needed

[CapePizza]

I have a 32 inch oven. The outer render is totally cracked and separating from the perlcrete layer. This spring I'm going to remove and replace the outer render.. I thought, while I'm at it, maybe I'd add another layer of perlcrete. My question.... any idea how much perlite I'd need for a 1 inch layer with a 1:8 mix ratio?
Thanks!