We study the probability density distributions of mass estimates for class II methanol masers with linear position-velocity structure, of maser cluster, using Monte Carlo tecniques. A uniform random distribution of maser sources along an edge-on rotating circumstellar-ring is assumed as a working hypothesis. It is found that in the cases with sufficiently high number (> 6) of observed maser spots it is possible to set upper limits for the mass encircled by the ring. Both turbulent motions and observational uncertainties are shown to result in the broadening of the probability density distribution of the mass estimate. As a consequence, commonly used procedures do not provide correct values for the lower mass estimate. Ten maser objects with the number of observed spots exceeding 6 were chosen for preliminary analysis. Within the framework of the working hypothesis all of these objects were found to be massive (M > 5 M⊙) with negligible probability for the mass exceeding 80 M⊙.