Sounds Like Good Advice

[T]he "extreme programming" planning technique: you estimate work by chopping it up into tasks and figuring out how long they would take in "ideal days" (that is if everything goes perfectly) . Then you see how many "ideal days" worth of tasks you managed to deliver last month (for instance) and multiply your estimate by the ratio actual days/ideal days.

chris cottee, comment on "Planning Fallacy" by Elizer Yudkowsky