Set some interim goals, estimate them and measure progress. Take that progress and estimate the remaining work. There's no accounting for the unknown, but IMO i

Set some interim goals, estimate them and measure progress. Take that progress and estimate the remaining work. There's no accounting for the unknown, but IMO it's best to raise the alarm as soon as possible. Just make sure you and the team have the ability to deliver the project, otherwise it's time for a different conversation where you look for outside help. As much as everyone hates deadlines they are important and try your best to meet them if they're realistic.