We present a new technique for calculating the fraction of dark material within the optical radius of spiral galaxies. The method employs the well-established observational result that spiral galaxies have similar central surface brightnesses, as well as published stellar synthesis evolutionary models, color-magnitude relations and optical rotation curves. No assumptions about the dark matter distribution are necessary. We find that the ratio of disk-to-dynamical mass within the optical radius increases roughly as L B 0.4 . This is in good agreement with the results of Persic and Salucci which are derived from independent considerations