Funding payment and final PnL of a BTC perpetual

I am attempting to determine the funding payment and final PnL of a BTC Perpetual contract.

In the formulas used by a few Perpetual exchanges the Impact Notional (Average execution price for a market sell and buy) appears to be close to the Index price of BTC. My question is how often is this Impact Notional adjusted? Can the Impact Notional be thought of as the average execution price for a market sell and buy of a market order of 1 BTC?

Secondly in attempting to determine a trades final PnL, am I correct in assuming that since the price of a futures contract and the spot price converge on the delivery date, the final PnL is just the relationship between the opening price and the closing price, multiplied by the leverage. For the purposes of final PnL we can ignore all of the price movements in-between, making

Final LONG PnL: exit price - entry price (multiplied by leverage) Final SHORT PnL: entry price - exit price (multiplied by leverage)

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