I'm guessing the SRR is etched in the ground plane (?).
In this case, I would consider a normal incidence test. Excite a plane wave normally to the SRR and observe at which point the reflection phase is ~0 -- while not strictly correct this method is used all the time in the literature.
Alternatively, observe the transmission properties through the SRR when illuminated with the same plane wave (you can get the S21 from this process). Of course, this implies a surface, not a material, but again the difference is usually glossed over in the literature.