Well, 70 degrees is probably closer to the right answer, but not because that's the conditioning temp!
When you use those calculators, they take into account that colder liquids hold onto co2 better than warmer liquids. A colder fermentation temperature would have more co2 dissolved in it. So, in order to not have overcarbonation, you'd use less priming sugar. However, if you fermented at say, 52 degrees, and then lager the beer, more co2 wouldn't appear later at lagering temperatures. Or if you ever had the beer at a warmer temperature, the co2 would start to come out.
The correct way to use that calculator is to use fermentation temperature, or the highest temperature the beer was kept at. That would be more accurate. The difference really isn't that much (in the dissolution of co2) between a lager and an ale. So, don't worry about it too much.
A diacetyl rest is useful to encourage the yeast to digest diacetyl. Yeast have characteristics that will produce the presursor to diacetyl as a byproduct of fermentation. Not only do yeast produce the precursor to diacetyl, they also consume the diacetyl produced, and enzymaticly reduce it after fermentation is finishing.
Sometimes diacetyl isn't much of a problem- either that strain isn't a big producer, or the primary was long enough that it got "cleaned up" in primary, or due to pitching enough yeast at the right temperature, not much was created. However, in a warm temperature lager yeast pitch, or a rushed primary, or in a yeast strain that is notorious for diacetyl, it could be an issue.
Diacetyl in small amounts has an oily mouthfeel, or slickness on the tongue. In large amounts, it tastes like butterscotch or buttered popcorn. A diacetyl rest near the end of primary will encourage the yeast to clean up the diacetyl. After lagering, probably not. The yeast responsible for cleaning up the diacetyl will be gone and/or way too tired to get to work.
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