You will not have NanoDrop-able concentrations of RNA from 200 ul of starting material. (You will get a reading, but it will not be reliable.) It is recommended to normalize downstream assay loading by volume. In my lab, we generally check the concentrations of several endogenous miRNAs as well as that of a spike-in to assess recovery (by qPCR).
You will never be able to have an accurate RNA concentration extracted from biofluids with the nanodrop. The amount and concentration is too tiny for this machine. You will need to use something like Qubit or Bioanalyzer to have a correct quantification.