When your steem power reaches a certain limit (I think around $500 or something), a slider appears in the UI that you can use to determine the percentage of your vote. That slider can set the percentage of vote to integer percent values from 1 to 100. If you need a finer gradation, you can use the [`steem.js`](https://github.com/steemit/steem-js) library that allows you to set your vote weight on a 10,000-point scale.