Hi Aamir,
Follow precisely the current path. If you have a 1k in series with 50k and current is 100uA you will have a problem as you will be outside the voltage compliance range for the IDAC.
For the 50uA case I believe this should work, but your measurement is not ratiometric as you are using the internal reference voltage. When using your analysis you must be careful about the precision based on using a specific current. A ratiometric measurement takes the current out of the equation and you actually measure the ratio of reference resistance to the RTD resistance. The way that you are making the measurement you measure a voltage and assume a current. As you have already determined the current may be off. As per the datasheet, the initial current error can be as much as 6% of full-scale. The value you measured with the amp meter is within this tolerance and is actually pretty close to what I would expect.
Best regards,
Bob B