Gaping necklines are my absolute pet peeve. I find them so annoying that most of the time I do a preemptive neckline alteration before even trying on the muslin. I just know my neckline will gape, as anyone with an above average cup size will know. Even after an FBA your neckline will still gape. So you can either tape down your neckline....
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.... or just make a super simple, very tiny neckline alteration to shorten the neckline. So what will it be? Fashion tape? Neckline alteration? Fashion tape? Neckline alteration?
Well, here is what you do if you decide to alter your neckline:
Lengthen the bust dart of your pattern to reach the bust point. Then draw a dart from the neckline to the bust point. You can find out where your bust point is by either tissue-fitting or fitting a muslin to yourself. Mark your bust point and move and/or lengthen the dart to meet the bust point. You can determine the size of the neck dart by fitting a tissue pattern or muslin to you as well. Simply pinch away as much as you need, secure with a pin, mark the dart and then measure it and transfer the measurements to your pattern.
Once you have marked your dart, slash the bust dart and fold away the neckline dart. Your bust dart will increase in size and the neckline will look oddly misshapen. Add some tissue paper to your dart (and accept that it is now even bigger and more unwieldy than before) and redraw the neckline. (If you only need to shorten the neckline by a tiny bit you can simply fold away a tiny the dart and squash the paper a little, without actually cutting up the pattern and shifting the dart around.)
Voila! You are done!
For other styles, like princess seams, you should be able to shift the neckline dart into a seam. Sometimes you can also easily pinch away fabric directly at the seams so there is no need for shifting at all.
I hope this helped some of you big-busted and hopelessly neck-gaping ladies out there. As you can see there's no need to despair. Neckline gaping has easy remedies. Good luck with your alterations!