Here's my swatch. . .
I measured it in several places for two or three inches and calculated that my gauge is 4 stitches per inch. Elizabeth Zimmerman insists that we are absolutely honest with ourselves when we measure a gauge swatch--really, no cheating! Notice the pins in the fabric? I placed one pin at the edge of a stitch, laid my ruler on the swatch, and then placed a pin in the fabric two or three inches away. Then, the ruler is moved, and I count my stitches, and divide by the number of inches I've measured. Moving the ruler away from the measurement prevents you from cheating and sneaking a stitch to one side of your inch line or another.
I repeated the process for my row gauge. This may be an important step in your calculations for this sweater--don't leave it out!
After looking through my stitch dictionaries, I thought I'd settled on a simple lace pattern that always seems to look nice. I was committed to Old Shale (or Feather and Fan). I like the bit of ridges in the pattern that will look nice with the ridges where there is an increase. I was ready to cast on.
But wait. One of the nice things about that lace pattern is the wavy edge. On the original pattern, there are increases in the garter ridges--that would adjust my wavy edge, and I think make the lacey area look like crap. Back to the stitch dictionaries.
I found a drop stitch pattern in my vogue book that I like. Twelve row repeat, a bit open, nice clean edges. . .perfect! Now for the measuring.
A good friend and an honest look at your body may be required at this point. Measure your fullest bust point, and write that number down somewhere. Mine is 44.5"--don't spread that around, okay? Now measure around your fullest point, and include your arms, that is, hold your arms at your side, and measure around your bust and arms. . .my measurement is 55". Then place the tape around your neckline. . .how open do you want this sweater to be? How low do you want it to hang? A nice snug neckline to keep you cozy in the winter? A drapey, low and open neckline? You make the call, measure that circumference and then measure the depth from that opening to your fullest bust point. My neckline is intended to be at my collarbone. . .about 25" in diameter. . .distance from collarbone to fullest point of my bust? 9 inches (we'll call this the bodice length).
Go have a cup of tea. Really. Math is coming next.