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You should keep your voting power between 80 and 100%. If you take a look at the table that was posted in the comment by @themanwithnoname, you’ll see that you can upvote 10 posts at 100% to make sure your VP doesn’t go below 80%

If our vp is below 80%, what is the impact of our account?

The lower you go, the less value your vote has.
Somebody please correct me if I’m wrong, but when your voting power is 80, it will recharge back to 100 in a day, so you’ll be able to vote at 100% again.

If you do 11 votes at one time, you should be able to get 11, not 10...but the VP of each will be lower than 100%.