(optimistic time + (4 times more likely) + pessimistic time) / 6

A further enhancement is to perform only the pessimistic timing analysis.

Accurate estimate= (Optimistic time + 4 Must likely time + pessimistic time)/6

The Expected Duration field uses the Duration2 custom duration field.

Pessimistic time (a): worst time expected if everything goes very badly.

You can enter the expected duration as part of a PERT analysis calculation, which takes a weighted average of expected, pessimistic, and optimistic dates and durations, or you can have Microsoft Office Project calculate it for you.

When you enter expected start and finish dates, and then click Calculate PERT on the PERT Analysis toolbar, a probable duration is calculated and placed in the Duration field.