I would recommend to plot a Campbell diagram, to make sure the operating speed and harmonics are not to close to the natural frequencies. The possible excitation frequencies have to be determined depending on the type of driver, and also based on the design of the fan(blade pass frequecy...
I use a software (ARMD from RBTS company in the US) that calculates the coeff you are loking for.
I also have a batch of curves giving the coefficients versus the Sommerfeld number. Much cheaper !!!