When selecting a size a lot of patterns (mine especially for the Linear Fit) will direct you to use your full bust to select a size.
And that works great for a frame that doesn't have a lot of extra girth in the front.
But if you have large breasts and select a size that way you may end up with the front shirt fitting okay but the back maybe a little too big.
That's because the girth is evenly split between the front and back and that's not going to work with your frame because your girth isn't evenly distributed.
So how do you or I pick a size?
Well, we can try to select a size based on our frame and then edit the pattern to accommodate our bust.
Below I'll show you using a raglan as an example. This technique can be used for set in sleeves as well. Cut through the armscye (armhole) on the set in sleeve style and the rest is the same.
Step 1. Wear the undergarments you would wear under the garment you're making.
Measure your upper bust (above the breasts). Use this measurement as your "chest measurement" and select your size*.
Print the pattern piece out and make any length adjustments (add or subtract length) you need.
*Remember our direction for selecting a size was to use our full bust. We're using our high bust (upper bust; measurement taken above the breasts).
Take the front pattern piece and lay it on your upper chest with the fold mark at the center of the chest.
The pattern piece will flair out and away from the front centerline.
The distance from centerline is the amount of width you'll need to add to the front pattern piece. Write this measurement down.
Before removing the pattern piece mark the apex (often this point is where the nipple is located).
Step 3. Draw 2 lines.
Line 1 should be from the side of the front pattern piece to the apex.
Line 2 should be from the bottom of the front pattern piece to and through the apex to the middle of the shoulder seam.
Cut line 1 to the apex but not through, leave about 1/16" of paper to create a hinge.
Cut line 2 to the apex and then pivot to the shoulder seam, leave about 5/16" of paper to create a hinge. Make a 1/4" cut at the shoulder seam to line 2. Don't cut through to line 2, this is your hinge.
Step 4. Tape the pattern piece down at the center line.
Separate the pattern piece (section 2) in the amount of width you measured in the previous step.
Make sure section 1 and 2 are parallel when measuring the gap.
Step 5. Slide a couple pieces of paper under the pattern pieces.
Fix the pieces so the gap is correct again and then tape the pattern pieces to the paper.
Step 6. Trim off the excess paper.
Note: Any width added to the front will also need to be added to the waistband. Example: If my gap was 2" I've added a total of 4" to the width of the front of the pattern piece. I'll need to add 4" width to the waistband.
Step 7. True the side seam of the front piece. If the front piece's side seam is longer than the back by 1-1/2" or more you will need to trim the excess from the bottom of the piece.
Subtract 1-1/2" from the excess and that will be the amount you will trim. (Example: The front side seam is 2" longer than the back. I will subtract 1-1/2" from 2" and now I know I need to trim 1/2" from the bottom of the front pattern piece.)
Measure up from the bottom of the pattern piece and mark.
Draw a line from the side seam to line 2. Blend the line to remove sharp corners.
Step 8. Trim the excess paper off. You're now ready to sew up a muslin.