Once you have 500 STEEM Power, you get an "upvote slider" when you click the upvote button. You can give 1% to 100% (in increments of 1%) for your vote. Before you get to 500 STEEM Power, every upvote is a 100% upvote.
While doing this I recorded the upvote percentage before, after, and the dollar amount of the upvote. You can see your upvote percentage here: https://steemnow.com/
Enjoy!