Wearing that dress convinced me that girls with low busts can enjoy a bust shelf as long as you mentally relabel it as a rufflely bust dickie. It's there to cover your bust root and fill some your large upper chest area with fluffy goodness. To that I say, "Thumbs up to fluffy goodness. Let's sew some more bust dickies."
To give you a colorful mental image of what I'm talking about let's think of the breast as a cupcake. The bust shelf is the ruffled cupcake liner with the top half of the breast, the icing, sitting above it. This is all well and good if your apex matches this point and you have plenty of "icing" to fill out the top of the bodice. But what does this bodice look like if you have a low bust point? Or to further our metaphor, your breasts are more like creme filled donuts than cupcakes? (Just as delicious but with less fluff on top) What you are going to get is a bodice with MAJOR gapituts along the top edge of the inset.
Sorry about the poor quality photo but I was a bad girl and threw this muslin alway some time in the last 3 months. You can still see that I have literally inches of extra fabric along the top edge since all my bust fullness in in the lower part of the bodice. What you can't see is that the front part of the lower bodice feels tight because that's where my apex actually is. My face also says, "Crap, I need to do some major fitting work on this pattern."
That muslin might make the pattern look like a lost cause, but never fear because the bust ruching is going to allow us to make an easy fix. All we really need is elastic! That's right ladies, we're gonna sew elastic into the upper edge of the bust shelf and make all our problems go away. Woot, Woot, fitting short cuts! But first let's make few minor adjustments to the front bodice.
First you can remove a bit of that extra ease at the upper edge by changing the curve of the bust stay. This will keep the lower part of the seam the same width, which is good because your boobs are in that area. On my dress I was also changing the location of the seam, so some of the width removed here was added onto the bodice piece. (Note - I changed the height of the bust stay for aesthetic reasons, not fitting ones.)
On the lower bodice you'll want to add some extra ease to the princess line seam for your low bust point. I added 1/4" to each side of the seam. (You can also see that I added a bit to the outside curve and changed the length of the bodice. The lengthening was more for portional reasons than bust fitting reasons since my bust won't actually hit the shelf area.)
Then you may want to reduce the width of the bust inset. On this pattern the ruching ration is fairly high on the bust inset. Adding elastic to that area is only going to increase it and it may get poofier than you'd like. Of course this is more of a personal taste than fitting issue, so use your own discretion. On my dress I reduced the inset by making the same curve change to it as was made on the bust stay. Then made the pattern smaller by moving the CF fold line in and cutting off some of the ease. In total I removed 2 3/4" on the half.
To assemble this area follow the pattern instructions for gathering the bust inset, sewing it to the lower bodice and the top edge to the bust stay. Before you do any edge stitching on the top edge, open the bodice like so.
We are going to sew in some 1/4" elastic onto the seam allowance to tighten up the area. On my dress the inset area was 13" wide without elastic. I wanted to gather it down to around 10" to get a nice snug fit against my chest. After much trial and error, meaning sewing and seam ripping, I ended up inserting an 8" long piece of elastic. Long story short, I recommend basting in the elastic and checking the fit before "really" sewing it in.
The elastic is going to be basted into the seam allowance of the bust inset area. First fold the seam allowances the same way you would for stay stitching. Pin the edges of the elastic near the edge of the bust seam. I found it's a good idea to baste both ends of the elastic first before attaching all of it.
Stretch the elastic to that it sits flat against the inset and baste it to the seam allowance. At this point try on the dress to see if the upper edge fit is to your liking. If not you can rip out the elastic and try a different length.
Once the you're happy with the elastic length, it can be secured with some edge stitching. On my dress I edge stitched the entire upper edge of the bodice at once, but you could just do the inset area.
Go back and remove any basting left over. Then press the edge so that the lining rolls toward the inside of the garment
Ta Da! A perfect fitting bodice. See that actually wasn't all that hard to do. Thanks elastic!
* Note about cup sizes on this pattern - The shelf pieces vary 1/4" in height between each cup size but are the same width. I did go with the recommended cup size for my bust, but don't think it's as important for women with low busts. You could just choose the height of the shelf you prefer the look of the most.