Back when Burdastyle was Burda World of Fashion, they published a method for sizing their patterns up or down. Most of the time I fall into a range where I need to size many of the regular patterns up or size the plus sizes down, so I've used this often and feel confident about grading up to two sizes different. Beyond that, I'd be sure to make a muslin to check that the pieces go together as they should, just in case there was an incremental error that got compounded.
Here's a quick illustration and explanation of how to do it. In the photo, I am sizing down the pink pattern piece by one size. (It's the center back piece of a princess seam garment.) I've made my markings in black. Ignore the black curve labelled 5 on the right...that's another pattern.
Draw lines connecting all sizes at the seamline intersections. They will be diagonals at all kinds of angles. Extend those lines into the interior of the pattern piece if you're going smaller, or to the outside.
Now eyeball or measure the distance between the sizes and start to draw your new seamline the same distance from the closest size. You want to echo their lines - so notice that the black line I've drawn for the neck starts out farther from Burda's line at the center back fold, but gets closer at the shoulder, just like all the other sizes do.
When your line reaches the next diagonal that you drew, that's where you will change direction and start another seam line.
Remember to add your seam allowances before you start cutting.
This post was written so I'd have a photo to share on Pattern Review to answer a member's question. It's such a simple concept, and not hard at all to do, but difficult for me to put into words. Maybe it will also help someone else.
