Nearly all successful stories fall into the three act structure; the first act is the setup and definition of a problem or pivotal change in the characters life. In the first Harry Potter novel, ACT I consists of Harry turning eleven, Hagrid coming to his house at that moment to inform him he is going to Hogwarts, and Act I ends when Harry boards the magical train (chapter six; on page 116 of 584 in my copy, 19.86% of the way through) and enters another magical world.
This event was set up by Harry turning eleven, on page 56, the end of chapter 3 (9.6% of the way through, and almost precisely halfway through the first act). This is the life changing event for Harry, something happens to him that will change everything, even though he doesn't realize this on page 56.
In your story, you have (as DPT said) about 20% of the length to set up the story and central conflict. Your character's moment should begin at 10%, and probably peak around 15%, and then wind down the consequences by 20%, when the reader realizes that act will be "defining his continued existence".
The first 10% is to get the reader accustomed to how the character acts and thinks before the instigating event. Then 5% to progress to "the great victory at great cost", then 5% to deal with the consequences of "the great cost", the perceptions of fame and/or villainy, the emotional reactions of the MC.