I understand that for a tournament you need a blind structure. I found example in PokerSoup.com. Here are the inputs they have used. No of players = 20. Tournament length 4 hours. Smallest chip denomination 25. Starting chips 5000. Round length 20. I though there would be 12 rounds as tournament length/round length, i.e. 4*60/20 = 12. But as per example there are 20 rounds. I am also assuming there is no time lost between rounds. Here is the link for the example: https://pokersoup.com/tool/blindStructureCalculator
Tournaments will have a targeted time, but may generate a winner before or after that point based on how the participants play. You're not going to guarantee a stopping point to the minute.
I suspect that the blind structure that site creates has a "target" round when a winner is predicted, but includes a few more rounds just in case. All the on-the-fly and common structures it lists seem to include exactly three rounds after the target time (whether that's 3 more hours or 15 more minutes).