By the look of it it’s the big readers who are getting up to Regular first (20k+)*. And 50 visits seems to be the minimum. Don’t know if that’s of any interest to anyone? 
*Extra snippet. I think these may have to be from posts that were created within the 100 day window?
- Of topics created in the last 100 days, must have viewed 25% (capped at 500)
- Of posts created in the last 100 days, must have read 25% (capped at 20k)