Reading progress bar allows readers to define where they are on the page. If we say as an expert;
The principle behind this technique is based on a simple fact that the user needs to scroll to reach the end of the web page. Once the user reaches the end of the web page we can conclude that he/she has finished reading the article. Our technique revolves around the scroll event which is likely to be the key to determine an approximate position of the user while reading.
Example(circular form): Startups Weekly: Diamond-encrusted disruption | TechCrunch