How do you check your coverage? Or even how many words you know?
No idea about its accuracy, but as a native English speaker it's telling me my vocabulary size is 30k words, which sounds roughly correct.
Those are not set-in-stone science, so it is just an estimate, but I take about 10+ over those years, and the the number from those estimates seems consistent.
Coverage, there is some paper to track, I just googled it.