Future's Cash Settlement - Spread question

Hello all,

I have a question concerning Index Futures cash settlement.

Let's take the NDX 100 future. The spread for the closest maturity is usually 0.25 (ie Sep maturity) when the equity market is open. The future with a further maturity (ie Dec maturity) typically has a higher spread since the open interest is way lower.

When the Sep maturity future comes closes to maturity (a few days before), the spread starts getting wider. Already saw it at 0.50, but I guess it can get even wider right before maturity.

I would like to rollover the position while optimizing the spread. Therefore, one idea would be to close position when the Sep spread is still at 0.25 and open a position when the Dec spread is also at 0.25.

However, as an alternative, I was wondering if waiting for the settlement would be more advantageous. Let's say that the spread is of 0.50 1mn before the maturity. At settlement, what would be the cash I receive based on ? On the bid price ? On the ask price ? On something else ?

Thank you for your time.