There are softwares like Spider project that lets you develop your schedule utilizing 3 time estimates based on true pert method, probabilistic, optimistic and most likely. Utilising this feature you can cover your risks as contractor when you present your external schedule to the client you could present the probabilistic schedule (with considerable buffer compared to Most Likely and Optimistic Schedule), while internally you could utilize the schedule developed on most likely time frames, and put up a bonus incentive to project team to achieve the optimistic schedule.
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