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The Meninting Dam under construction on Lombok Island, West Nusa Tenggara, Indonesia, requires a good planning to build a diversion tunnel to support its development and mobilization. The diversion tunnel is planned to be built through rocks with medium to poor rock mass quality. The planning stage involves various parameters, i.e., the rock mass classification, using either the RMR or GSI method. Converting values from one method to another makes planning work easier. However, the constraints found were the limitations of the observational data, such as discontinuity conditions. The objective of this article is to discuss the alternative depiction of discontinuity conditions in rock mass using RMR method. An alternative equation was developed to obtain a prediction model for determining the RMR value, based on GSI data. The evaluation showed that the mathematical models developed in this research had a small gap of error compared to other values. The models then can be used to predict RMR value based on GSI data and vice versa, with a higher degree of accuracy and precision according to the actual rock surface conditions, especially in the construction site of diversion tunnel at Meninting Dam.