Your best bet is to go very
Your best bet is to go very slowly (50 degrees an hour or so) starting at about 1050°F and creep up to about 1225°F.
You should also make sure your kiln/shelf/mold are level.
Even when you do everything right, deep slumps often drop catawampus. Very often the piece can be repositioned in the mold and slumped a second time.
One more option is to make your bowl blank bigger than needed and put a drop ring above the mold — then cut the rim after slumping. That’s how I did the bowl in this forum thread here.
Helios Kiln Glass Studio