Flattening before Slumping works Best
I agree with Terri in that you may acheve a better result if you “flatten” the bottle with heat before a slump firing. The temperature you listed in your firing schedule is sufficient to flatten it, but lengthen the time of the hold (by 5-10 mins depending on your kiln). Each kiln is different, so test fire & take notes. Use the schedule that produces the best results for your aesthetic.
Your slumping temp doesn’t nearly have to be that high. Depending on your glass properties & size, your slumping temperature should be around 1200-1300 degrees and held for about 20 mins. Don’t foget to kiln wash your molds.