I am not sure what are you trying to create? Why do you need a post-reading percentage?
The blog system doesn't require any post percentage system. You should describe more about it. Just edit your question and more description.
You can create levels on words and paragraphs to reach a user on your page.
Level 1 for 20 percentage
Level 2 for 40 percentage
till the level n for 100 percent.(Bottom of content)
When the user crosses the first level then insert the percentage into the data with the user's IP address.
like - 20% (insert 20% )
then 40% reach (Update 20% to 40% after reach)
follow the same process with ajax and PHP. It all depends on scroll down and level when the scroll bar touch levels.