Viscous damping in BLDC
Ideally, for viscous damping the force (torque in your case) is a constant times the (angular in your case) velocity. 

You can calculate the coefficient from other motor parameters. 

To find the viscous damping torque at a given RPM you must convert from RPM to radians/second so a factor of \$\frac{2\pi}{60}\$ (about 0.105) is involved. Then multiply the angular velocity is s^-1 by the coefficient to get torque (Nm).