You just use the fermentation temperature, even for lagers. The reason is that co2 is "held" better in a cold liquid. But cold crashing after fermentation doesn't make more co2. If the beer was fermented at 50, then it would have more residual co2 than if it was fermented at 70, but once it's done fermenting it's not producing more co2 anyway.
So, if the beer was fermented at 64, use 64 as the temperature of the beer. The highest temperature that the beer was at is the correct number to use for the calculation.
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