To calculate unrealised profits and losses in future contracts, not always the last traded price of the future is used.
If the last traded price is not within the current best bid-ask, then the bid price or ask price will be used, whichever is nearer to the last traded price.
Further the mark price can never differ more than 2% from the Deribit BTC index (for the weekly future, currently the only future traded on Deribit), or when in last half hour before expiration, the estimated delivery price. These rules will prevent liquidations due to manipulative trading.
The Mark Price for calculating profits/losses in future contracts will be 271.50
The Mark Price will be 271.20
The Mark Price would be 280, but because the difference between the index and 280 is more than 3%, the Mark Price will be 270+3%=278.1
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