Ok, there’s two points I’d like to make about what’s going on here. You know how the strap starts out on a weird angle? It’s basically following the outer edge of the side-front piece of a princess line draft. No joke.
(That’s never been really obvious, because we didn’t draft above the bustline.) When we rotate the strap to a vertical position, we’re closing the big gap of negative space between the Center Front and Side Front pieces.
Second thing: Your strap, as mine, might extend slightly above the top bar of the T when you rotate it. That’s good, because we want the chemise a little higher than where a bodice would hit. If it doesn’t knock an inch or two off the strap (for a human – use 1/8″-1/4″ for a doll).