There's an "upvote slider" that appears when you click the upvote arrow -- but, you only get that after you have achieved (or purchased) 500 STEEM Power.

I don't see that when I click. Perhaps I don't yet have enough steem power to do that?

Right, exactly, you need 500 SP for the slider to show up.

If you know coding, you could use the command-line tool which would allow you to specify the percentage of the vote, even with less than 500 SP.

Also, I think the phone app "esteem" will also allow you to specify the percentage [edit: with less than 500 SP, that is], but I haven't explored it much and haven't used that feature -- I'm mostly here on my laptop.