It’s just a skirt, right? In this particular case, I’d do a size 16. For bottoms, you want to choose based on your hip size and then you can use our increase waist pattern alteration tutorial to add 1″ to each side seam, which will increase the waist from 30″ to 34″. I don’t know what kind of skirt it is, but given the increase in finished garment measurements, it sounds like it’s for design ease, or the style of the skirt isn’t meant to be skin tight and have some room and movement. So if you want to take advantage of this, then just stick with size 16. Hope this helps 🙂