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The payout value directly follows the market cap. Coinmarketcap is a bit distorted though, they substracted Steemit's stake from the total amount. Real market cap is at 26.8M at this very moment.

And an increase in market cap would add or reduce the value? I assume reduce?

No, add of course :D In the end, you receive Steem in one form or another. That amount is decided on by the blockchain, dependend on the votes. When a Steem is worth more, your payout is too.

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