This post is for alice930
I know the pictures are a little mickey mouse-ish but bare with me;-) I think this is what you where asking. I hope it helps.
Measure where you want to add material to. Say it was 30 inches around. Now divide that by 2 because that's what you're going to use at the top of the vertical stripes. 30/2=15 Now you know how many stripes you will have, 15.
So with sweater you will have 15 vertical stripes on the skirt, right?
I use 2 inches at the top of the stripe , 5 at the bottom
Remember to add seem allowances so it will be 2 1/2inches at the top and 5 1/2 at the bottom when you cut it out. And maybe 15 inches long.
So the waist will be 30 inches but the flare at the bottom will be 75 inches. Don't be afraid to play with the numbers at the bottom. the higher the number the bigger the flare.
I like the look of the horizontal stripe at the bottom of that. It's also a nice way to add extra length.
Note: - heavier materials can be MUCH harder to work with if you're using a serger.
- make sure you double check your measurements.
I hope that helps :-)