The amount is determined by the SP that each upvoting users has in combination with the percentage of a 100% upvote that a users allows for the upvote on your post, combine that with the constant fluctuations of the STEEM price and the whole thing gets very confusing.
Also many users will only upvote content that is selected as 100% SP in the rewards section for posts. My 392SP gives me an upvote I can give of 0.02 if at 100% voting strength (which I'm not)
Wow nerd...lol.
Seriously though, that formula seems like it makes sense. It has to do with tenure, your current SP and rep, and whether you have 100% vote or not. So many variables it makes one's head spin.