After you have tack fused your stringers, you will have a fragile blank, about four stringer layers thick. This will need to be handled gently and watched carefully during slumping.
When you slump, you should fire very slowly. Although thermal shock is not likely, the thin pieces have a tendency to crack when they bend into the mould. A safe firing schedule would be to advance at no more than 150C/hr to 540C, then increase the temperature at about 55C per hour toward 677C. Keep watching until the piece slumps into the mould, then advance to the next segment of the programme and anneal as usual for 6mm thickness.
For your first attempts, it's a good idea to use a shallow mould. After you get a feel for the process, you can achieve deeper slumps.