This study investigates the behavior between Finite Element Analysis (FEA) results and the modified Westergaard equation, taking into consideration of the boundary effect on stress distribution around the loading area of a mechanical part with limited dimension. The Westergaard equation, a definitive problem-solving method in elasticity of pavement with ‘boundary at infinity approximation’, serves as the theoretical basis while a modified advancement is introduced to optimize parameters and address differences between theoretical assumptions and FEA results. The study emphatically included boundary effects in the analysis, acknowledging their effect on stress patterns. Parameters, such as stress measurement horizontal location (r) and stress measurement depth (Z), are systematically modified, and a new variable, load-to-boundary distance (LB), is introduced to achieve a more accurate alignment between FEA and the newly tuned Westergaard theoretical results. This finding noted the relevance of the original Westgaard model error as well as the boundary effect in tuning the original Westergaard equation, contributing to real-life engineering applications, especially on mechanical parts with limited dimensions. More specifically, the original Westergaard model is tuned by introducing polynomial constants (A and B factors) to correct its error in the application of mechanical parts with a single material. The boundary effects of mechanical structure with limited dimensions and its induced stress distribution error can be described by a Gaussian function with variable, LB.