Enter your target keyword in the search box and check the stats of the videos on first page, you'll see data like x views from youtube search for <tag>. Write the highest value down and go to the next video, until you cover all the videos on first page. If you don't think you can reach the first page, then do the same strategy but for the second page or third page instead of first.
The neat thing about this approach is that if you know that, given your current resources, you'll end up say somewhere near the middle of page 3, you can then accurately estimate the number of views you'll get with ease. On the other hand if you picked tags randomly using youtube keyword tool or from random competitors, you can never be sure how much traffic you'll end up getting.