Thanks to Rod McGregor, a service tech in Australia, who shared his method with us.

Walking Belt Length
L = 3.142D + 2C
L is the walking belt length,
D is the diameter of one roller (assuming identical diameters)
C is the distance between the roller axle centers (front to rear)

Walking Belt Speed
S = 0.24RL S is the belt speed in KMH
R is the number of rotations of the belt in a 15 second period
L is the belt length in meters
For a 30 second count period use S = 0.12RL