You get the vote slider when you are near 500 SP. However, if you use it right away, your votes of course will be worth less, so you must make a choice... keep giving 100% votes at 4 to 5 cents each or drop the amount in order to give more votes. I prefer to keep my vote value as high as I can since the value is already so low. The trouble is that I run out of VP too quickly and can't give out as many votes as I would like to! Anyway... I have a way to go to get my slider, so in the meantime I'm always running out of vote power.

I THINK busy.org offers a vote-slider to all users, so you could use that portal and have a slider now (I may be mistaken). Unfortunately, I find it a bit non-intuitive to use, so I continue using steemit.com as my portal to the blockchain.

With the price of Steem at close to $1, it seems that your vote is worth roughly 1 cent for every 100 SP.