When we insert a time buffer in a schedule we are trading cycle time for a reduction in cycle time variability.  This looks sensible until you do the math and realize that the cost of the buffer virtually always exceeds the economic value of reduced variability. It is usually a mistake to pay for variability reduction using one of our single most valuable performance parameters: cycle time.   A more economic approach is to substitute variability in a cheap performance parameter for schedule variability.  Analyze which project parameter has the lowest cost of variability. You may discover letting expenses, unit cost, or even performance deviate from plan is much cheaper than inserting margin in your schedule.  We refer to this method as variability substitution, and it is usually more economical than using a schedule buffer.