LOL, MT!
Julia's calculations are pure guesswork, unless she had a spare moment one evening to calculate the volume of the coffin, minus the space taken up by the lining and by Barn.
DS Wiki has a shot of Barn in the coffin. We can see that there's a chain around his neck, but can't see the cross. Maybe Julia was giving herself and Willie a little insurance in case Barnabas did revert. In later scenes, it's gone.
Carolyn takes Adam to the West Wing, even though Stokes doesn't think it's a good idea because he suspects (correctly) that Barn will now be desperate to find Adam. Stokes can't come up with a better idea, but he's just as smooth an operator as ever. When Julia arrives at the root cellar, Stokes fibs that Adam ran off into the woods again, explaining blandly, I am not agile enough, nor is Carolyn strong enough, to detain him.
Stokes delivers his famous quote: I wish I knew the secret of Barnabas Collins. He inspires so many emotions in so many people. I intends to find out that secret, too, he promises as he leaves. But as far as I know, he never does.
DS Wiki also has a nice shot of Barn's anxious eyes as he looks at his reflection in Julia's mirror, wondering whether he'll be (literal) toast at sunrise.
The scene with Barn and Angelique at sunrise is terrific! It seems that both JF and LP are having fun. This is the first time we get to see just how bad a winner Barnabas is as he gloats over Angelique, then leaves her fuming.