I'm not sure how others calculate these in their heads. I do it more off experience. I've been given over the yrs a few formulas and why they occasionally worked they really didn't seem reliable to me. So I canned them.
The 12th exacta seems a little on the low end of where I would estimate. It is a Ronnie Wrenn horse on top that though. If I had that exacta for $2 I'd have thought I would get $75. But the payoff doesn't seem way out of whack to me. Just my opinion. I've listened to a lot of folks dispute these over the yrs and imo people usually project the high end of where the payout will be. Just an opinion though- I know people have strong feelings on payouts and I respect that.