What values should I enter for stiffness and mass proportional damping coefficients?

Using mass and stiffness proportional damping results in a critical damping ratio that varies with frequency according to:

...

\;\;\;\;
\left{
\xi_i
\xi_j
\right}
=
\frac{1}{2}
\;\;
\left[migration1d:
\begin{array}{cc}
\frac{1}{\omega_i} & \omega_i
\frac{1}{\omega_j} & \omega_j
\end{array}
\right]
\;\;
\left{
\eta
\delta
\right}

...