It’s an awesome idea to track progress like this. Indeed, number of sentenecs sentences on its own doesn’t mean much and it really needs a context like this. If you asked me to guess, I’d say an average book has at leas 10k words — never really thought about it.
What might not be a great idea is using HP and sticikng with it. It makes it look like there is some connection between the site and HP. Maybe let us choose a popular book in profile, or just grab some “average book length” statistics, although that might be bringing up questions like “what is average” and we’re back at start.