hi,
If you extract R2/R1 only from rigid part of your protein, That's sound good because TauC should increase significantly with oligomerization.
Exception can append, depends on structural state of your protein, for exemple : a random coil monomer have aproximatively the same TauC as his associate globular homodimer...
If you want to be sure about oligomerization, try a size exclusion chromatography or a dynamic light scattering....