The payout your vote gives is not strictly linear. It follows some quadratic function. It's very easy to notice, vote on a post with 3000$ of payout already and see how much it goes up, a ton. Vote on a post with $0.00 payout and it'll barely if even go up.

I'm not sure what value that site is using to determine your voting power, it's probably something in the middle range so it'll be a good estimate but know that the payout a vote brings will vary a lot.