I went to a conference
Having that kind of focus from the word "go" would help you write your book, as well as helping you to sell it later! Try this exercise right now, no matter where you are in writing your current manuscript. It's tough, isn't it?!
Now of course, plans change. Perhaps your vision for the story changes as you write. You find a new and stronger direction. Maybe your initial idea wasn't strong enough. Deep enough. Intriguing enough. So of course, your twenty-five word distillation of the story idea would need to change. If so, then go with it. Type up a new one-liner, one that helps you focus on the core idea of your story, and keep it above your computer.