Reducing the ever-growing leakage current is critical to high performance and power efficient designs. We present an in-depth study of high-level leakage modeling and reduction in the context of a full custom design environment. We propose a methodology to estimate the circuit area, minimum and maximum leakage current, and maximum power-up current, introduced by leakage reduction using sleep transistor insertion, for any given logic function. We build novel estimation metrics based on logic synthesis and gate level analysis using only a small number of typical circuits, but no further logic synthesis and gate level analysis are needed during our estimation. Compared to time-consuming logic synthesis and gate level analysis, the average errors for circuits from a leading industrial design project are 23.59% for area, 21.44% for maximum power-up current. In contrast, estimation based on quick synthesis leads to 11x area difference in gate count for an 8bit adder.