Depending on the level of accuracy required, you could test a given length with freshly cut ends and reduce the length as appropriate to get resistance per unit length data for regression / statistical analysis. Repeat for several samples from different bars/production lots if possible. Properties may vary across different manufacturers if the type of steel alloy varies.
I would thing that with the surface irregularity, the mix of materials that can go into rebar and with the deposition of these on the outside surface that you might get some 'scatter' in the results. I wouldn't be looking for any real precision.