Thanks! @remlaps came up with a really creative way of calculating efficiency for paid out posts. He did the ratio of curator_vests to total_vests from the payout transaction over the ratio of curator_rshares to total_rshares in the post.

(curator_reward_vests / total_vests)
~-----------------------------------------------
(curator_rshares / total_rshares)

I thought this was a very smart way of calculating the efficiency without having to convert vests (since the conversion factor is always changing). I think in this case, 100% would represent the rewards the curator earned being equivalent to the amount of value they provided to the post, and more would be the curator earning more than they contributed, and less would be less.

I'm not sure if this efficiency is comparable to your efficiency, but I think it is a good metric for evaluating my ai's performance after payout.