The stability graph method is a widely used empirical method for dimensioning open rooms/stopes and support design based on stope geometry and stability number. This paper presents a rigorous investigation employing numerical models featuring multiple joint sets to replicate the structurally controlled failure of open stopes. The equivalent linear overbreak/slough (ELOS) in the model is calculated based on the volume of the detached blocks identified by defining thresholds for normal and shear displacements on joint planes. Modified Factor B values are proposed based on analyses of numerical models with varying critical joint set angles. It is found that the numerical simulation results align well with stability graph outcomes when using modified Factors A and B. This research has improved the understanding of the empirical stability graph method and promotes its reliability in predicting the stability state and unplanned dilution in open stopes.