Lunchbox tee view b dropping armholes

I just sewed view b for my 9 year old who is broad across the shoulders and very very pre boobies. The size 12 with 1.5 inch length added does fit her but seems it would be more giving if I moved it a bit off the fold and dropped the arm pit a bit to make bigger arm holes. I think it would pull a bit less. How would I adjust the size of the cuff relative to the amount I drop the arm pit?