Tolerance - is the permissible deviation from a project plan within which there is no need to request a decision from the next level of management.

The standard tolerances that must be managed are time(schedule) and cost(budget). If you manage your project within the tolerances you have been assigned your project will be a success.

To a large degree this just means identifying and managing the risks that impact on time and money. A clear head and some objectivity can certainly help for this as you may be a risk yourself :)

The relative importance of project factors that rob_au lists might help find the factors that will most affect your schedule