> For example, my squad completes 85% of stories in 5 days. Given that, the number of stories in the epic, and number of engineers, it's easy math to calculate

> For example, my squad completes 85% of stories in 5 days. Given that, the number of stories in the epic, and number of engineers, it's easy math to calculate an approximate completion date. Until it turns out that the remaining 15% takes half of the total time. In my experience, estimating about 80-90% of a project accurately isn't that difficult; it's the other 10-20% that can end up taking considerably longer than you expected and throwing off the entire estimate.