Optical Proximity Correction (OPC) is still nominated as a main stream in printing Sub-16nm technology nodes in optical micro-lithography. However, long computation time is required to generate mask solutions with acceptable wafer image quality. Intensity Difference Map (IDM) has been recently proposed as a fast methodology to shorten OPC computation time with preserving acceptable wafer image quality. However, IDM has been evaluated only under a relatively relaxed Edge Placement Error (EPE) constraint of the final mask solution. Such an evaluation does not provide a satisfactory confirmation of the effectiveness of IDM if strict EPE constraints are imposed. In this paper, the accuracy of IDM is deeply analyzed to confirm its validity in terms of wafer image estimation accuracy along with its efficiency in shortening computation time. Thereafter, the stability of IDM accuracy against the increase in pattern area/density is confirmed. Finally, the regions suffering from lack of accuracy are analyzed for further enhancement. Experimental results show that congestion in the mask pattern forms a cardinal source of the lack of accuracy which is compensated through optimized selection of the kernels included in IDM.