Currently, the design flow of digital integrated circuits relies on the use of pre-characterized libraries. These libraries, pre-calculated outside of any context, are used at each step of the design flow to predict the performance of the circuit. They are necessary for the calculation of a wide variety of characteristics, including the IR Drop. However, the behavior of the logic gates described in these libraries is not representative of the real power supply of the standard-cells and this difference is even more important as the number of logic gates in the circuits increases. In this paper, we will show that with the current design flow, the use of these libraries leads to an underestimation of the circuit performances. To remedy this, we propose an iterative approach based on a new use of voltageadaptive libraries that allows to compute the IR Drop more precisely and thus to better predict the performances of the circuits.