I actually had to grade the front piece out a couple of sizes from the largest due to my Vast Acreage and I wasn't convinced that it would be an effective method. My thoughts were probably "how on earth is this going to work".
My horizontal dart is between the bust points, as my main issue was a gaping neckline. I'm sure I've seen it on something period, which is why I thought of it. I could have eased the neckline onto a stay tape but that seemed like a lot of work & would be easier in wool than cotton, and easier in a V or U neckline than in a square.
Wishing you fortuna in getting it to work without much more angst