This plugin estimates the time it would take for a slightly below average reader to a slightly above average reader to read your post. It then applies
The estimation is based on a span of 150 words per minute (slightly slower than average) to 250 words per minute (slightly higher than average). These numbers can be changed easily from within the code of the plugin (lowered to account for more complex content and raised for simpler content). Future versions of the plugin may allow for the changing of these parameters via an "options" page.