As technology advances to 45 nm node and below, the induced effects of etch process have an increasing contribution to the device critical dimension error budget. Traditionally, original design target shapes are drawn based on the etch target. During mask correction, etch modeling is essential to predict the new resist target that will print on the wafer. This step is known as “Model Based Retargeting" (MBR). During the initial phase of process characterization, the sub-resolution assist features (SRAF) are optimized whether based on the original design target shapes or based on a biased version of the design target (resist target). The goal of the work is to study the different possibilities of SRAF placement to maximize the accuracy and process window immunity of the final resist contour image. We will, statistically, analyze and compare process window simulation results due to various SRAFs placements by changing the reference layer used during placement.