Take it up to slumping
Take it up to slumping temperature. You probably will have to watch, as bottle glass requires higher temperatures than fusing glass. I have done it at about 720C.
You do need to be careful about annealing though. At the base, the glass may be 13mm thick when slumped. So your annealing needs to begin about 80-100C below the slump temperature with a soak for about an hour, followed by a slow cool, say abour 30C/hr to about 400C and then 100C to room temp.