Your development agency says the project is 80% complete
Your development agency says the project is 80% complete. Before you feel relieved, ask what is being measured.
Usually the answer is tasks. Eighty percent of the tickets are closed. That tells you the team has been busy. It tells you nothing about whether the remaining twenty percent are the twenty percent that matter, and they usually are, because teams do the parts they understand first and leave the integration, the migration and the exceptions for the end.
The twenty percent that is left is where the unknowns live, so it is where the estimates are worst.
Ask a different question: which of the real jobs can a user complete today, end to end, on our data? Enter an order, ship it, invoice it, report on it. If the answer is "the order screen works," the project is not 80% complete. One job out of four works, and the hardest three have not been attempted.
A better measure of progress is jobs a user can finish, not tasks a developer has closed. Put that in the contract as the milestone definition and "80%" stops being a feeling.
The honest answer to "how complete are we" is a list of what works, what does not, and what has not been tried. Any single number is a summary designed to end the conversation, and the conversation is the only thing protecting your money.