This paper proposes DECBS, a novel approximation algorithm for the Multi-Agent Path Finding (MAPF) problem. MAPF is an NP-hard problem where multiple agents find a path to a goal without collisions. Existing constrained approximation algorithms, such as ECBS and EECBS, utilize focal search techniques to balance computational efficiency and solution quality. However, conventional focal search suffers from the problem of slow growth of the lower bound (LB) value in the early stages, limiting the search space. DECBS addresses this problem by first determining the maximum LB value and performing a best-first search based on this value. Experimental results show that DECBS outperforms ECBS in most cases and is compatible with existing optimization techniques. Specifically, when the agent density is medium to high, DECBS achieves an average 23.5% runtime improvement over ECBS under the same suboptimality bound and optimization conditions. It also reduces high-dimensional CT nodes by approximately 30% and low-dimensional focal search nodes by approximately 50%.