We evaluated the optimal harvest timing for olive flounder (Paralichthys olivaceus) farmed in Jeju Island, which accounts for ca. 51% of P. olivaceus production in Korea. Typically, stakeholders determine their harvest timing based on their personal experience rather than considering operating costs, profit, and biological factors such as growth and mortality. This approach often leads to biases, and wrong decision. To address these problems. We developed a data-based decision-making framework for fish farms. We discounted monthly cash flows at 4.5 percent and expressed them as equivalent uniform monthly worth to compare schedules of different lengths. Across evaluated schedules, equivalent uniform monthly worth (EUMW) ranged from 0.23 to 0.57 million KRW for harvests starting at months 6~9 (starting weights 0.31~0.63 kg) and ending at months 16~21 (ending weights 1.73~2.69 kg) under the high-mortality (70%) scenario, and from 5.02 to 6.33 million KRW for harvests starting at months 23~34 (3.10~5.26 kg) and ending at months 41~43 (6.45~6.76 kg) under the low-mortality (24%) scenario. This study highlights the importance of quantification in aquaculture decision-making, demonstrating its potential to optimize stocking strategies, inventory control, and harvest planning. By applying data-based approaches, we expect fish farms can enhance economic sustainability and long-term operational efficiency.