How should I determine the percentage of my Vote Strength ?

If you can use busy.org or eSteem Surfer, you will see a voting slider at the time of giving upvote. Then you can easily determine the percentage of your vote strength.

You can also see voting slider on Steemit if you have more than 500 SP. If you have less than 500 SP, you cannot see voting slider on Steemit. And you have to give upvote using 100% strength.

Now if you have less than 500 SP and you want to determine the percentage of your vote strength, just use busy.org or eSteem Surfer to upvote. If your SP is more than 500, you can determine the voting strength anywhere.