Once you have the heat rate q_r you add the resistance of the new wall to the Sum of thermal resistances, then you use the emissivity to calculate the new rate of heat that is coming out of the new wall (this one should be minor than the original q_r). Then you have the net value of the heat rate that is going to be in direct contact to the fluid. That way you have q_{new r}= Delta(T)/Sum(Thermal Resistance), beacuse Delta(T) is T_{fluid in the middle} - T_surrounding, you can find now T_surrounding. T_{surrounding} = T_{fluid in the middle} - Sum(Th. Res.) * q_{new r}