WES 2805, a Japanese fitness-for-service procedure, proposes a crack tip opening displacement (CTOD) design curve for the brittle fracture assessment of welded joints, where the local strain acting on the cracked region, ε, is calculated using the stress concentration factor, Kt, and ε is converted to CTOD. However, in WES 2805, Kt, is shown as a few constants that depend on the ratio of crack depth, α, to the welded joint thickness, t, and the constants can be excessively conservative. In this study, the relation between Kt and the stress intensity magnification factor, Mk, was theoretically indicated, and the utility of Mk in the design curve was investigated for three kinds of welded joints. In addition, the local average strain by area was calculated by finite element analysis, and was used for the design curve instead of ε. It was demonstrated that constant Kt was sometimes excessively high and Mk was helpful in modifying Kt. Moreover, the local average strain by area contributes to good CTOD estimation, especially for a toe crack in a load-carrying fillet welded cruciform joint.