Personally I always go with the chest measurement to determine a size. That measurement determines fit not only across the chest but also across the back and the armscye (armhole for sleeveless or sleeve size). In the case of Jamie if you go with the waist size it measures a 1X which is 2 sizes larger than her chest size, which is a size L. You would need to decease both the chest and across the back as well as the armscyes so the top is not baggy with armhole openings extending down the sides and being baggy all over. Keep in mind also that plus size girls usually are large around the chest measurement but many times do not have much of a cup size, making the darts and fitting all off for patterns with fitted tops. The waist size is easier to adjust and the hip size is not important in that pattern. Have you considered a pattern with a less fitted top? You could do a peasant style and then wrap a wide waist or empire level tie which would be flattering. You could go with the same fabric or a coordinating one, or a different pattern/color on each side of the sash. This type of pattern would be fun and youthful with far less flitting issues. Sis Boom has several in that style, such as the Dana pattern and you could wrap a wide empire sash on that in a coordinating color which would give it a more fitted appearance. Just an idea, but if you are up to doing alterations (several of the O+S sewists are quite advanced in their skills, I am personally not one of them!) and she is available for a few fittings then you could use any pattern.