The three inputs
Deliverable count. Each named output has a production cost, including the boring ones like exports and specs. Unknowns. Anything that needs research or a technical spike gets a range, not a number. Dependencies. Third-party assets, approvals, and legal review are counted as calendar time, not work time.
Why estimates come as ranges
An estimate with one number pretends the unknowns do not exist. A range tells you where the risk sits.
Narrowing the range
The range narrows as unknowns close. We restate it at each phase boundary rather than letting the original number drift quietly.