This is the easiest bust dart adjustment EVER! In these photos, I’m wearing my Beatrix muslin with dots representing where the dart ended before the adjustment (closer to the middle) and after the adjustment (closer to the sides).
The darts are at a good height, and the size is right, though, so the only thing I wanted to do was shorten the dart by an inch. Generally, you want the dart to point in the direction of your bust apex, but ending 1/2″ – 1″ short of the apex. (Rae adds: typically the 1″ is for bigger busts, 1/2″ for smaller busts)
Here’s the Front Bodice pattern piece from Beatrix, traced in my size. All I had to do was make a new point on the dart’s center line one inch away from the original point, then re-draw the dart legs using a straight edge, and starting at the same points on the side so they’re the same length and they don’t change the side seam.
If you need to lengthen the dart, simply extend the center line by the amount you need, and re-draw the dart legs exactly the same way.