Peter spends several days on figuring everything out, though the last few of those days are mostly for the sake of being Responsible and excess of zeal than because he thinks he'll change his mind about anything. He's talked about it extensively with his friends and with his sister, a little bit with his brother (but not that much, because as expected Philip just wasn't very interested), somewhat more with his dad, and not at all with his mom. He'd in fact rather not think about that whole affair if at all possible, it was sufficiently unpleasant he managed to be negatively surprised, and he doesn't want to commit a description of it to narration. His mom is still alive and hasn't had any heart attacks at all nor committed herself to any institutions and that's the best he can say about it.

The notebook can't tell him what will happen to him once he finalizes everything because he's going for the Isekai Roulette and so that'll all be up to the Spirit. He doesn't know how long he'll be away for, or when and how he'll come back, or any details like that. He's preparing for it to be A While, and his loved ones will miss him and he'll miss them, but he will be back, he will, and they will all be happier at the end. They will have to be, otherwise this wouldn't be a story he'd want to be in.

He decides he'll vanish from a public place, a park, with enough people around that they'll see it happen but not so many that they'll just bystander effect their way into believing it never happened or was someone else's problem. He thinks the story where a protagonist sees this happen and investigates it and finds their own avenues into the supernatural would be really cool and fun, and he wants to enable that. They probably won't be a Mary Sue, themself, but hopefully something will happen.

Or it won't. He doesn't know. He's not going to try to influence it one way or another any further than his vanishing act. Let the story that will be told from that point be free to go where it will, as long as it's not awful.

So that's where he is, a park, and that's where he confirms to the notebook that yes he's sure of his choices and he's ready to go, and after a cool magical girl transformation he's somewhere else.