To enhance the efficiency of supply chain management, it is essential to reduce safety stock levels without compromising product availability. To achieve this objective, it was proposed to evaluate effectiveness of managerial decisions based on fill rate calculations. Fill rate is a common measure of the customer service performance of inventory and is often presented as the percentage of units available when requested by the customer. The findings from several studies remain contentious; for instance, increasing the order quantity may offset a reduction in safety stock levels, or vice versa. Furthermore, our research reveals that existing approaches occasionally produce inexplicable outcomes, such as negative values for the fill rate or safety stock levels. The proposed approach adjusts existing methods based on the consideration of inventory management strategy with periodic or constant control period and proposes a methodology that allows managers to control the variable level of safety stock to maintain the desired value of the fill rate.