How to handle bookmarks in Reflowable books

I am in the team who builds an eReader which supports Reflowable books and books are in ePub format.
When I launch the Reflowable book the page count is dynamic in nature.

For Example:
Given I have book that has 100 pages on default 100% screen size
When I zoom in to 200 percent, the page count increases to 200 pages
When I zoom out the screen to 50 percent the page count decreases to 50 pages

In this scenario how can a user bookmark the page, because the content always flows in the book. If I bookmark page-2, and I zoom in the page-2 content will be scattered to 3 and 4 pages. I am not sure how a user can bookmark pages. Hope the answer works across all the zoom in-out percentages.