Hi @cmp2020, there is still no good solution for retrieving the historic values available. As I was asked this question a few times already, I'm planning to provide a new method that can offer a better/faster way in the near future.

I was curious about how you and steemit convert curation rewards in vests to steem power.

The conversion from VESTS to SP just uses the current values from the blockchain. As we receive rewards in VESTS, whose worth increases over time, we acually earned more in terms of SP than what was shown in the past. Hard to say, if it would be more correct to always use daily conversion rates.