b-y, m1, c1, Hbeta  ->  Mv
Dambis, A. K., 1991, SvAL 17, 307
Calibration of B-Y/0 Color Indices and Absolute Magnitudes M/v for A4-F3 Supergiants 
using Uvby-Beta Photometry

[m1] = m1 + 0.33*(b-y)
[c1] = c1 - 0.19*(b-y)
   r = 0.35 * [c1] - ( Hbeta - 2.565 )

Valid for:
  0.12 < [m1] <= 0.23
  r > 0.2
 
Mv = -9.23 + 21.0 * ( Hbeta - 2.55 )

standard error = 0.32mag