The table in the back of the book is really helpful, and you should also follow the guidelines about hip splits (namely that you don't want to have more than an inch difference between the hip measurement in the apron the hip measurement in the pleats and that you want the bigger measurement in the pleats). So,

hips: 20 (apron) and 21 (pleats)

Then, you can't have more in the apron at the waist than there is in the hips, so the waist measurement has to be 20 or less. If you chose 20, the apron would be straight with no flare. Not ideal. If you chose 19 in the apron, you'd have 18 in the pleats at the waist. If you chose 18.5 in the apron, there would also be 18.5 in the pleats at the waist, again, not ideal.

So, I'd use:

waist: 19 (apron) and 18 (pleats)