The control signal sharing while packing flip-flops and other instances in slices is a necessary constraint in the placement of instances in FPGAs. Global placement usually does not consider signal sharing. In this paper, we propose a control signal aware slice-level packing algorithm within the framework of window based legalization method to obtain an optimized legal layout, satisfying all constraints, after global placement. We select a target window with the highest number of overlaps. Then, we check the capacity of the target window and adjust its size to secure enough space required for legalization. Lastly, window based legalization takes three constraints into account: 1) Control Signal Sharing: Two Flip-Flops in a slice must share a single control signal in FPGA architecture. 2) CLB Architecture Matching: Instances should be placed within a half slice to minimize the routing requirement. 3) Slice Level Packing: Instances are packed into slices for effective utilization of available empty space within a window. The experimental results show that our algorithm performs better with 45% less block displacement and 10% less runtime with the same wirelength when compared to a previous well-known mixed size block greedy legalization method [1].