Heater skin temperature estimation

Hello,

In my company, every time we have to model a piping system connecting to a heater, we are considering 200F skin temperate due to refractory.

Is there a more accurate way to calculate it? What is your approach?
Is Vendor obligated to calculate it from refractory thickness?

Thanks in advance

It would be reasonable to expect the vendor to give you the predicted skin temperature, but I'd say calculate the movements yourself from that data.