Mistakes to avoid
MS Project 2007 and 2010: building forecasts

Don't proceed "as if" the calculation of margins depended solely on the software. In version 2007 or 2010, the critical path and margin calculation are the result of "collaborative" planning that takes into account the context and risks of the project (external and internal) and defines the "realistic" (as in achievable) conditions for bringing it to a successful conclusion.

Don't forget that it's the project manager's responsibility to monitor any slippage in the critical path and margins as the project progresses. It is essential to measure their impact (on the project completion date and on the work of those involved) and to choose the actions to be taken to minimize them. Project delays and budget overruns are not inevitable!