At the start of chapter 19 what happens? We get a new story. John begins a new section to us.
"And after these things I heard..."
Chapter 18 must be split away from chapter 19's new story.
Chapter 16 is when John begins to reveal to us what was in the little open book that he ate in chapter 10.
16:1 links from where?
the end of Rev. 11
Rev. 11 gives him the info to show that another set of plagues is about to begin. That set will end with great hail.