Guessing the difference... the difference will vary with outside temp. At -10 the difference might only be 20 degrees while at +20 degrees the difference could be 35 degrees... The difference will vary depending on if there's a kid (or more) in there as well... The kid(s) would add to the warmth making the difference even greater regardless of outside temp. And you can't discount the kid(s)'s coat color either... a darker coat color will absorb more of the light's heat energy thereby raising the inside barrel temp even further! Then you have to consider orientation of the barrel WRT any breeze... The temp will vary on a logarithmic scale based on wind speed and angle of incidence. I expect humidity and barometric pressure to also have some influence. You see, -----
Hmmm maybe I am overthinking this? OK... ok... so say 26.75 degrees average
I think it's hard to predict when the doe will kid by ultrasound. I know quite a few goat and sheep people that had their animals ultrasounded and the vet(s) were way off on their predictions.
Stinks to play the guessing game when this if your first time with kidding!
Keep checking the ligs.