My solution is a combination of several different techniques I've tried over the years. It seems like a lot of steps but once you do it a few times, it only takes a few minutes. I've found the adjustments I need to make are consistent throughout a pattern line, but varies between different pattern companies.
For demonstration purposes, I'm going to show a 3 inch FBA (for a total circumference increase of about 6 inches, with all the extra ease in the front of the garment).
This is the pattern bodice front as it comes out of the envelope:
Patterns from the big four American commercial pattern companies (Vogue, Butterick, McCall's, and Simplicity) are drafted for a size B cup (independent and European pattern companies vary). If you're larger than a B, choose your pattern size by your upper chest measurement, not the full bust, so that the garment will fit your shoulders properly and so that the armhole is the correct size. As a general rule of thumb, you need to add 1 additional inch of width over the bust for each cup size over a B - so by adding 6 inches, we're redrafting for a G or H cup (depending on how your bra manufacturer does their labels). And a large bosom needs extra length as well as extra width.
1. I start by tracing the armhole opening (armscye) on the original pattern:
Set it aside; we'll use it later so that we can preserve the original sleeve/armhole fit.
2. I never make adjustments to the original pattern. It only takes a minute to trace off a clean copy and if you screw it up, you can always start over.
A standard 3 inch FBA results in a huge dart that's impossible to get to conform to the shape of the bust, so to start with, I add 1 inch to the side seams of the bodice piece. This reduces the amount of extra fabric I need to work into a dart:
4. Now tissue fit the bodice piece to see how much more width you need to add (do it on yourself, not your fitting assistant, of course). Because the pattern was sized according to the upper chest measurement instead of the full bust, you can see that it fits properly at the shoulder, but the girls are distorting it below that. Notice how the armhole is being pulled towards the center; if left unaddressed, this pulling makes the armholes feel too small when in reality they are the correct size. Also the dart points too high because there's not enough length to adequately cover my totally impressive front porch:
Cut along the grain line from the bottom of the pattern piece to the apex, then out to the armscye, stopping just before the armhole seam line. Snip the seam allowance, leaving a small hinge right at the seam line. Starting at the side seam, cut the center line of the dart to the apex, again stopping just short so that you have a little hinge:
If you measure between the two halves of the apex mark, you'll see that you've added length to the bodice - in this case, 1 1/4 inches. This is good:
Fill in the space with some tissue paper and trim off the excess paper from the edges of the original dart. Mark the center line of the new dart:
7. Remember how we wanted to preserve the shape of the original armhole so that the sleeve fits properly? Now's the time. Take the tracing of the original armscye and lay it over the altered pattern:
You've added some diagonal length over the girls (which keeps the girls from pulling the armhole forward and distorting the fit), but the shoulder and the armscye length have remained unchanged. The extra ease is shown below:
At this point, I make a clean copy of the pattern so I don't get confused by all the markings. You'll also need to shorten the darts so that they end 1 to 2 inches away from your apex:
8. Time to tissue fit again. You'll see that the pattern reaches the center front nicely:
and the side seam is pretty straight (it would be totally straight on somebody who still had a waist, but I don't, so it isn't. I always need to redraw the side seam so it goes straight down rather than angles towards the waist, but didn't do that as part of this demo because I thought it would confuse things even more):
At this point, you'd fine-tune the fit. If the darts still sit too high, you can make a standard lengthening adjustment above the bust, matched with a corresponding shortening one below it.
As always, make a muslin (test garment) before you cut into your fashion fabric, since you may need to tweak all this for your individual figure.
And, yes, I know we added about an inch to the front side seam length, but don't panic. The front won't quite match up with the back bodice piece, but I don't remove that extra length until I do a muslin. If you don't need the extra length at the side seam, lower the armscye by half the amount you need to remove and raise the waist seam at the side by the other half, redrawing the armhole and waist lines to make a smooth line (the increase in the armscye length will be negligible). However, I find that extra little bit of ease helps my clothes to fit well, because my girls curve out a little bit right there. I ease that extra fabric into the side seam at a spot level with my apex. You can just stretch the back piece a bit as you're sewing the two pieces together (put the longer front piece on the bottom, against the feed dogs), or if it's a natural fabric, run a gathering stitch and steam the extra fullness out.
Hope you find this useful.