What is the desirable thermal profile along the length & width of a billet (W=100mm x H=100mm x L=10,000mm) / slab (W=1000mm x H=200mm x L=10000mm mm) achieved at the end of soaking inside a reheating furnace if the rolling duration is 6-7 minutes approximately and at the end of last stand the finishing temperature is say 800°C all along the length of the slab?