The challenge is that there is always thermal resistance in conductors and thus a a temperature drop from one location to another. This is affected by the thickness, mass, dimensions, power available, thermal conductivity , temperature difference, thermal insulation and quality of assembly.
Ideally it can be as accurate as it is calibrated. In practice there are both the above mechanical and also electrical errors.
Do you want to understand how to design one, or just copy an existing one?