How I increase the cloth size of a pattern

In Full Bloom 06/2017 (A Burda Pattern)

So I used the same super easy pattern to make tops for all of my friends as presents for their birthdays. However I needed one top to be a size 20, which this pattern didn't accommodate for. In the past I have needed to increase the lengths of patterns however I have never personally needed to increase the size. So I thought I'd share with you how I went about this but in the knowledge that there maybe a better way to go about this. This being said, this method worked out perfectly for this pattern.

So starting on the side opposite the fold line, I measured the length of the pattern line for the smallest side. Then I measured the length of the pattern line for the next size up. I worked out the difference between the two. I did this for the difference between every pattern line and was glad to find out that the difference was the same for each size increase. Similarly the distance between each of the lines where the same.

Therefor the next step for me was to multiply these figures by the difference in size from the largest size on offer and the size I wanted. I used these figures to then draw out the new line.

I repeated this for each of the sides for the pattern. Like I said before, there is probably a better method for this, so if you have any hint and tips they would be greatly appreciated.