especially when instant experts cannot do heat transfer calculations
even halfway right.
Even if they could, it is a complex system with multiple components and multiple heat transfer paths, i.e. a real time waster, i.e.
P12 = k12c*(T1 - T2) + k12r*(T1^4 - T2^4) + f_naturalConvection(T1,T2)
P23 = k23c*(T2 - T3) + k23r*(T2^4 - T3^4) + f_naturalConvection(T2,T3)
P13 = k13c*(T1 - T3) + k13r*(T1^4 - T3^4) + f_naturalConvection(T1,T3)
P14 = ...
P = P12 + P13 + P14 ...
As many elements as parts in the reactor can be modelled. Big time to setup and solve. It is possible that Jed is correct and emissivity doesn't matter, but a definitive solution would take time.
R20 is the immediate goal. My preference is to change as little as possible between experiment and control, and that includes emissivity.