One management practice in is that you might estimate everything you do in ideal time. Ideal time is how long a task takes if there were no interruptions. And when you plan you don’t plan that all work is 100% focused and non-interrupted. Instead you can calculate with a velocity. Velocity is the number of user stories (or user story points) you can do in one iteration (or sprint) in Scrum this is known as the focus-factor. If your team (4 persons) under a 3 weeks sprint working 40 hours a week completes user-stories that where estimated to be 360 hours of ideal time your focus factor / velocity would be 66%.
Advantages of the velocity term is that it is much easier to estimate in ideal time, it’s what comes natural when you think of how long something will take to do. You get a focus on prioritizing to work with only planed work tasks. People avoid interruptions. You don’t need to plan to do more work than time available × the focus factor per iterations so you don’t get false expectations. You are able to trace the velocity over time, velocity has ability to peek in the middle of a project and become lower in the beginning and in the end of a project. In the beginning it can have with that people are stumbling on how they will work and they might not have all infrastructure and roles on place. In the end of a project new unplanned work have a certain ability to appear. It often takes between 3 to 6 iterations for velocity to stop fluctuate and become more stable.
This doesn’t mean that 34% of the time was wasted it, is not just spend on things that where planed and prioritized from start.