Estimate is but one part. The other is planning for obstacles and changes, i.e. have escalation paths and responsibilities in place:

Clear procedures to remove impediments, decision makers in the loop to ok scope/feature changes, resourcing agreed upfront, user engagement for testing locked-down, senior leadership aligned and kept informed regularly. Someone running the administrative side of the project, keeping people on target, etc. (could be a double hat, of course).

Sounds all rather "menial", but high degrees of organization really make a difference in delivery on larger projects.