To calculate the beta angle for pulley B in a frictional belt and pulley system, it is derived from the contact angle of the belt with the pulleys. The calculation involves subtracting the given angle of 60 degrees from 180 degrees, resulting in beta being 120 degrees. This approach assumes that the arcs from both pulleys' circles, represented as blue and yellow arcs, will sum to 360 degrees. The reference point for the angle is crucial, as it shifts from the top of the first pulley to the bottom of the second pulley, maintaining parallelism between the pulleys. Understanding these geometric relationships is essential for accurate calculations in pulley systems.