Original Benchmark Price

When do you differentiate that this is the first day’s close price or the relevant decision price that the person says is the relevant decision price? I’m specifically referring to the denominator of the IS equation, not the relevant decision price in Realized P&L

I feel like if the market is open when the decision is made, you use the decision price, but if the market is closed when the decision is made, you use the prior day’s close. Is this it? Or is it something else?

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I’m confused on this as well… believe i’ve seen it done differently in two different mocks. Anyone?