Estimate by comparing work to do with work already done
Obtain an estimate of the work remaining to do by breaking it into pieces that resemble work already done, and assigning a similar expected effort.
This pattern or a suitable alternative is a prerequisite for CompletionHeadroom. It may be that there are pieces of work that seem to have no comparable predecessors. It is likely that these will be higher risk, possibly architecturally significant. They should thus be candidates for prioritisation. When a team is just starting working together, there will be no preceding work to compare, and effort in the first tasks will be affected by learning curves. Thus estimates are likely to be poor or non-existent early in the project, but settle down once learning curves flatten and feedback on prior estimates is assimilated.