The logarithmic decrement lets you calculate the damping ratio. That would be easiest to use if you set the damping type to "Uniform Modal" where you can enter the Damping Ratio directly.
The Proportional Damping model needs coefficients to multiply by the K and M terms. Since the damping, C is calculated according to this formula:
[C]=α[M]+β[K],
you need to provide alpha with units of 1/s and beta with units of seconds to yield a damping coefficient of N*s/m or kg/s.
--Steve
