The NEC states table 9 was compiled using the Neher–McGrath ac-resistance calculation method, and the values presented are both reliable and conservative. The computation is not trivial, and is highly dependent on determining the conductor and insulating materials, and the ambient conditions. At the bottom of the table, it lists some of the assumptions.
Further analysis of the data between NEC and Revit shows that Revit is typically more conservative than the NEC. The table below shows that for a Power Factor of 1.0, Revit is slightly less conservative (that is, Revit will report slightly less drop than using data from the NEC). However, for power factors from slightly less than 1.0 down to 0.60, Revit will report greater drop (therefore, being even more conservative)... the delta between Revit and the NEC increases as the wire size increases, and as the power factor decreases.
As an example, in the worst case scenario for a 480V three phase circuit, 1000 MCM cable with 0.60 Power Factor, a maximum allowed feeder drop of 2% computed using the R value from Revit would result in a 1.71% drop compared to using the R value from the NEC. (Our R’s are bigger). Unless you know exactly the current in a feeder, and know exactly the ambient conditions, the best you can do is use an estimate, inheriting the assumptions such as those provided by the NEC or Revit. Either way, the result is conservative.