2 layers of glass
You will have better success with two layers of glass. Because of surface tension properties, glass wants to be 6 mm thick (which is two layers if you are using standard 3 mm glass).[Think about how water beads up when you put a teaspoon of it on a smooth surface.] Any glass stacked that is thinner than 6 mm will contract and any glass stacked so that it is thicker will spread (unless dammed). Using a single layer of glass is asking for dog-boning, unless you are firing at very low temps.
I typically do my slumps at 1225 deg f (663 deg c). I also incorporate a hold at 1100 on the way up. Quite often, by the time I actually hit 1225 (593) I don’t have to hold very long to finalize the slump.
That’s my experience, at any rate .
Jester’s Baubles Fused Glass Designs