How do I decide the value of damping ratios (frequency based) for determining Rayleigh's coefficient (Alpha & Beta) of proportional damping in the case of MDOF systems?

I am using method of modal analysis, followed by harmonic analysis to validate the mechanical design in one of my projects. I am using ANSYS Workbench 2017 and finding difficulty in deciding the damping parameters for harmonic analysis. My range of interest in frequency is 200 Hz to 2000 Hz based on Modal analysis results. I know the general trend of damping ratios which is, it shall increase with frequency (for my case of 200 Hz to 2000 Hz) but is there any fundamental method or source to find exact damping ratio which can be used as an input for my analysis…?