It sounds like you are trying to reproduce the work of McGaugh (2008, http://arxiv.org/abs/0804.1314 ).

In section 3, they adapt a model (the Tuorla-Heidelberg model), based on observations, for the baryonic mass distribution in the Galaxy. The model is based on a synthesis of a number of observational studies.

Stacy McGaugh has provided a series of tables that provide the mass density at a given radius for several scale lengths (2.0 <= Rd <= 4.0) on his web site - http://astroweb.case.edu/ssm/models/.