We are building a fire proof Safety box that can withstand a 700C fire for two hours and not allow the inside of the box to exceed a temperature of 150C using Transient Heat conduction. We are looking at one wall that is 1.5mm Steel, 21mm insulation, an 1.5mm steel again. We have calculated 32 nodes throughout the whole wall and have found the temperature of the 32nd node. What equation must be used to translate that temperature into the inside temperature of the box?