Those of you who have a full bust (C or D cup rather than B cup) know the frustration of sewing patterns and finding the finished garment to be too large in the neck, shoulder, and armhole area. Rather than choosing the size to accommodate your full bust, this is what you should be doing to get a better fit.
Measure your high bust, this is taken with the measuring tape in the same place on your back as the full bust, but raise the tape above your bust in front so that the tape is actually lying on your chest above the bust fullness. Pull the tape snug and use this number to choose your pattern size. So if you measure 42" full bust and 38" high bust, use 38" to pick size 16. Using your high bust measurement generally puts you into one or two sizes smaller than your full bust does.
Now, cut the pattern out on this size. Cut the sleeve this size as well. If the bust point is not indicated on the pattern, put the tissue against your body to find your bust point. Mark it on the pattern as indicated in the first diagram.
Next, draw three lines on the bodice front, as indicated in the following diagram. The first line comes up from the pattern bottom (point A) to the bust point and is parallel to the center front line. The second line goes from the bust point over to the armscye at the notch (point B). The third line is drawn from the side seam (point C) to the bust point, with an angle that resembles a bust dart, i.e. point C will be lower than the bust point and the resulting line will angle upwards to the bust point.
Now, cut from point A to the bust point, then cut over to point B, but do not cut through the tissue entirely. This needs to remain connected. Then cut from point C over to the bust point, but do not cut entirely through to the bust point. There should be a tiny bit of tissue left so that this piece of tissue remains intact. Now place some paper underneath your tissue, large enough to cover the area of all these cuts. Gently pull the tissue apart so that a gap opens from point A to the bust point. Keep the cut edges parallel to each other and pull the tissue apart until it gives you the extra width you require. Now this will have to be determined by you. If you need 2" extra across the front of the pattern, the gap at point A should be 1". The general rule is that you spread the tissue apart 1/2" for a C bust cup, 3/4" for D cup, and 1" or more for D cup and larger. I generally spread this 1" for myself because I don't want the dart at point C to get too large. If I need extra width beyond what has been added here, I then add some extra at the side seams. This will require some experimentation on your part to find out how much you need. And each pattern will differ, as some designs require less ease than others.
Now look at the bodice piece you have altered. You will need to extend the front hemline from the side seam to the center front, giving more length to that middle section. This is actually good because a fuller bust requires more length to go over it. Haven't you noticed that your full bust makes garments rise up at the center front? therefore this alteration will give you back the length you need there.
Next, you will need to redraw the side bust dart. It is probably pointing below the actual bust point. Keep the points on the side seam the same, but raise the legs of the dart so that they point to the bust point, but make the dart end 1 - 1 1/2" before the bust point. You don't want the dart coming right to the bust point or you will get unsightly points on your garment.
Now fold the dart down as it will be sewn. Cut the extra tissue from the side seam. When you open the dart back up, you will have those little angled pieces of tissue extending beyond the side seam. This is called "trueing" the dart, and will ensure that, when you sew it, the cut edges of the fabric dart will be matched up with the raw edges of the side seam.
If you have to add more width to the pattern as in diagram 3, this will create a very wide side dart. You can reduce the large side bust dart and make two darts. These will be easier to sew and will give better shaping to the bodice that one large dart. To make a second dart, fold out the side bust dart to the size you would like it to be, perhaps half of what it is now. Then slash the pattern up from the bottom edge towards the bust point and let the tissue spread apart. It will open in a dart shape. Draw your waist dart, with the legs of the dart beginning at the opening of your slash, but ending the dart 1" from the bust point. This alteration is necessary for women who have D or larger bust cup sizes, as one dart will simply be too big to sew. It will look too large and will produce a pointy bodice. Two darts distributes the shaping into two areas and the result will be a much better fitted bodice.
Once you have learned to do this alteration, you will find that you can use it on many patterns. It means that you can cut a smaller size for yourself in tops, getting a much better fit in the shoulder and upper chest area, yet having enough fabric to accommodate your larger bustline.
This is a very useful technique to know when making sleeveless tops. Cutting a size that is too large for your upper chest results in gaping armholes, because the armscye is too large for you. The excess fabric protrudes and you can pinch it out in a dart shape. Cutting a smaller size and adding a bust dart will correct this problem for you. If you are sewing a knit garment, you may not wish to have a sewn dart in the garment. In that case, you can simply make the alteration but don't sew the dart. Instead ease that part of the front bodice to the back bodice as you sew the side seam - this is possible to do with knit fabrics. Personally I don't mind having a dart in a knit top if it gives me a better fit.