Collision avoidance is essential for the safe operation of vessels, particularly in the context of emerging autonomous ships and advancements in artificial intelligence technologies. However, establishing collision avoidance standards remains challenging due to the absence of objective criteria. Article 17 of the International Code for the Prevention of Collisions at Sea (COLREGs) does not provide clear guidelines for triggering collision avoidance cooperation. Hence, Lee and Furukawa (2022) proposed a new collision evaluation model known as computed distance at collision (CDC), which enhances collision risk assessment between vessels. This study aims to improve the accuracy of collision assessment by adjusting the reference coordinate (RP coordinates) to align with the GPS antenna position, overcoming limitations of the existing CDC framework. The improved CDC model was applied to scenarios in Busan New Port and Tongyeong Port, quantitatively deriving precise timing of collision avoidance actions between two vessels. The outcomes of this study offer valuable data to assist VTS operations and enhance safety in maritime navigation.