The AD971x daatsheet states that the internal CML resistor can be adjusted from 250 Ohm to 1 kOhm.
However, Figure 102 shows a variation from 300 to 1100 Ohm. Which one should I consider for the calculation of the output common model level?
250-1000ohm is the nominal designed value, with switch and on-chip routing resistance, it is somewhat higher than that. So 300-1100 is probably closer to the reality.
Also, IC processing has great matching between devices within the same die (often much better than 1%) but poor absolute value from lot to lot (as much as +/-20%). If you want the CML value to be much better than 20% accurate, you may want to monitor the CML, and adjust the setting until it is what you want.