If you have trouble fitting the bust of your bodice or blouse, you may need a Full Bust Adjustment.
The FBA adds a bit of room through the bust, without making the entire bodice larger.
Most commercial patterns are made for a "B" cup body. If you're larger or smaller than that, your bodice may pull across the bust (if you're a "C" or larger) or gap (if your bust is an "A"). Your armhole may not fit properly, or you may have drape lines from the shoulder.
Even when drafting your block using your own measurements, the bodice darts are based on a "standard" number, which would work best for a "B" cup. Simply put, the instructions can't anticipate the actual depth of the dart for every individual, so the depth used in drafting is simply a starting point. That's why we do fittings!!!
You may find, in your fitting, that the waist of your dress block is pulling up in the front, while sitting properly at the back. That's an indication that you need an FBA - the fabric has more girth to cover from the shoulder to the waist (going over the fuller bust), and isn't long enough in the front only. In the case of a smaller bust, you may have excess fabric at the waist, although it will be most noticeable at the bust.
To make the bust adjustment, you'll be slashing a copy of your bodice block in several places, then spreading the pieces to allow for more coverage over the bust (or overlapping the pieces for a Small Bust Adjustment). For a full bust, the front bodice will get longer as well as wider, while for the small bust, it may get a bit shorter.
In any case, the adjustment must be made at the bust, not at the sides. And the final shape of the bodice may be a little weird, especially at the armhole!
But don't worry too much about that. Re-cut the bodice in muslin and do another fitting. I'm sure you'll find that it fits much better!
While I have a page explaining the FBA, it can be a bit confusing. I hope this video helps!
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