Lowering the waistband for a taller girl

So, my Ava is 12 (nearly 13, boo hoo!) and a bit tall for this pattern. Initially, I thought I would just lengthen the hemline about 3", however, the more I looked at pictures and the drawings the more I think it might look a little out of proportion on her. She is long and lean. She is so lean that it makes her look taller than she is. She is almost 5'2" and she likes her hem a little below the knee. How would you go about lowering the waistband a bit to keep the proportions of this dress looking right? Would you add length to the back bodice, then lower the front waist gathering line by the same amount? Or would you do something different? I'm afraid my modifications will not allow the waistbands to match up correctly. My sewing math stinks! I would appreciate any advice!