Modelling a temperature measurement system
In this problem you are asked to consider an old-fashioned (D'Arsonval, perhaps) mechanical meter movement attached to a thermocouple. (as an aside, with a bit of bimetal or a manual trim to do the cold-junction twiddle that was once a common type of thermocouple meter- aka "pyrometer"). 

The meter is fitted with a scale that indicates 20,000 degrees per volt of input. So it would read exactly correctly (assuming the 50uV/°C is correct)  if the meter did not load the thermocouple. 

In answer to your questions, you have the equivalent circuit correct. The error will be proportional to the temperature reading in this simplified problem, so Tm/T is a reasonable way to describe it. 

The voltage across the meter is a simple voltage divider, which you should be able to calculate. By inspection the error is of the order of -5% (but not exactly -5%). I'll leave that to you. 



In actual practice, this type of thermocouple meter would be designed for a specific resistance of probe + wiring (often something more like 10 ohms), and that requirement would be marked on the meter face. So a known amount of loading would be compensated for in advance via the factory calibration.