In Futures Contracts, How Is the Effective Spread Calculated When Trading a Large Block?

When trading a large block of futures, the effective spread is calculated similarly to other derivatives. It is twice the absolute difference between the execution price and the mid-price of the market's best bid and offer at the time the order was submitted.

However, for block trades, the execution price is often negotiated privately and reported after the fact, potentially allowing for a price that is better than the visible order book, thus narrowing the effective spread.