First we'll consider the case for which most patterns are written: tubluar person. (Tubular person is facing to the right, so that you can appreciate her/his tubularosity.)
Note that if the front and the back of tubular person's sweaters are the same length, they come to the same point on her/his body. Right now we're going to consider this the goal, though you can use these same techniques to make the sweater whatever shape you want.
Now we'll consider pointy person.
Pointy person's sweaters don't fit so well if the front and back are the same length. (See left figure.) In fact, pointy person is liable to end up with weird bunching and an unintended crop-top effect in the front of her/his sweaters. If this isn't the effect that pointy person prefers, then pointy person might be better served with a sweater where the front of the sweater is the length of the arrow on the right. (This arrow suggests a relatively clingy sweater.)
The situation is, naturally, a little more complex than this. When we're consdering bust darts, we really only want to increase the length of part of the front of the sweater by that much. Thus, for situations like these (which are pretty much all of 'em), one needs two measurements to add short-row shaping:
- How wide? Measure the width of whatever body part you'd like to cover at the most prominent part. For bust darts, this doesn't mean to measure the width of your breasts. This means to measure the distance between your nipples (assuming that they're the part of your breasts that sticks out the farthest. If you're actually making bust darts, you may not want the bust dart to become an arrow pointing right at your nipples, so add "a little" length to this. (I just use a measuring tape and start moving out until it doesn't look like I'm grabbing myself.)
- How long? As in the diagram of pointy person, measure length down to some point over the most prominent part of whatever body part you're trying to cover along the path you expect the sweater to take. Then measure length on the other side down to the same point. For bust darts, that means measuring over the fullest part of the breast down to something like the end of your ribs and then down over your back to the same height in back. If your sweater is going to be really clingy, hold the measuring tape close to you and right up under your breasts. If your sweater is going to be less clingy, drape the measuring tape the way you expect the sweater to fall (which is probably not right up underneath the breasts).
Now, for the math. This is really simple: knit a piece of fabric the shape of the red quadrilateral in the diagram below, then keep knitting your garment over all the stiches. It'll act like there's a triangle of fabric missing, just like if you made a dart in a sewn garment, and you'll get a graceful addition of length to cover your body.
More specifically, you can tell how many short rows you need by multiplying the length by your row gauge. Call this x Then the difference between the original width and the desired short-row width, times your stitch gauge gives you the number of stitches that you don't need to knit by the time you get to the top. Call this n. Since there are two sides, that's n/2 stiches per side. Because you only hit each side once every two rows, you knit n/2 divided by x/2 fewer stitches each time you turn. That's n/x stitches fewer each time. If it's not quite even, fudge mercilessly.
You can spread out these short rows in two different ways: separate them in chunks or spread them out diffusely. Separating the short rows into two chunks is especially useful if you have a very long length and/or a very short width. The math for this shaping is just like the math for a single set of short rows, just repeated upside down. :)
If this diagram looks a bit familiar, that's because it's exactly how short-row sock heels are constructed. You can also calculate the short rows you'll need and spread them out over a larger area to make them less obvious. Just keep in mind that if they get too far from the area that requires them, it can cause odd stretching in the knitted fabric.
Don't forget: this technique isn't just for bust darts! This is how you can cover more complex shapes (such as the following)
knit short-row heels for socks, or make the shoulders of your sweaters (especially raglans) fit better!