1. A method of determining a useful skew for a circuit design, comprising: a) computing a slack value for each sequential cell in the circuit design; b) identifying modifiable sequential cells in the circuit design, and computing a target delay for each modifiable sequential cell; c) discarding one or more sequential cells based on the slack values; d) determining a target slack value for a remaining sequential cell; e) comparing the target slack value with previously determined target slack values to determine a minimum target slack value; f) repeating d)-f) until the target slack values for all the remaining cells has been compared, and g) determining a delay for each cell based on the minimum target slack value.