Perhaps this example could be a useful model: https://boffosocko.com/2020/02/13/55767168/
The progress portion is coded roughly in HTML with a label as follows:
<ul>
<li class="bookprogress"><progress value="177" max="465">38%</progress> <label for="">38.0% done; loc 4290-4847 of 12932</label></li>
</ul>
You could always use <p> or <span> instead of ul/li tags (with some app specific classes to allow the receiving site to create its own custom CSS for display. Otherwise browsers should be able to display a reasonable visual default.
I’d recommend support for pages, percentages finished, and potentially even Amazon’s default location numbers, with the ability to translate back and forth potentially when given at least two of the parameters as a minimum which should allow the calculation of the others. I find in practice that it’s generally pretty rare to have both page numbers and location numbers, but it could happen.
I’ve also got an extended version available at https://boffosocko.com/2012/06/17/big-history/#READING%20PROGRESS