The way it works now is that if you start with 100% voting power and you upvote someone at 100%, then your voting power will be depleted by 2% (2% of 100%). However, let's say you upvote a second post immediately after. Well, then your voting power gets depleted by 2% of the remaining 98%, which makes it slightly less than the original 2% out of 100%. The more you upvote, the less of a bite it takes out of your voting power. Meanwhile, your voting power replenishes at a rate of 20% (always out of 100%) daily. So that means you can never run out of it, so you should upvote as much as you want.

To get some experience, go to steemd.com/thephotoguide. In the left hand column, you will see a line item for your voting power. It is currently at 96.79% but will be different than that when you look at it. Then upvote something on Steemit and then refresh the steemd page. You will see a log of your action at the top of the right hand side of the page, and you will note a decrease in your voting power. Make a few more upvotes, noting what's happening with your voting power each time. Then go to sleep and check voting power again the next day and you should see a significant increase. You do this a few times and you'll get an intuitive sense of how it works. Hope this makes sense. Cheers!