Need help choosing size

Hello! I'm sewing Cappuccino tunic for my aunt and need help with the size. Should I choose the size based on high bust or full bust for this pattern? High bust is size 4, full bust size 8 and she has B cup (5cm difference between high/full bust). Also her waist is size 16 but hip is size 10. So should I use size 10 or 16 when grading?