If all you are doing is slumping, 787C seems high.
Also 515C seems low for an annealing soak, since most bottle glass behaves pretty much like float.
I presume you do more than just switch off after the hour at annealing soak. Depending on how thick the piece is I would consider 80C/hr to about 400C to do the annealing, before cooling faster.
Still, it is only a bottle.