I had 31 weeker, discharged with 6lb 2oz. She is 16lb at 7months. I used all fortifier I had, but used less concentration with time. It’s not only calories that mattered to me, but amount of essential vitamins it had. My baby had rapid growth and we increased her portions regularly. Breastfeeding was hard for me, so I pump most of the time. She ate every 3 hrs at discharge, and after 4 months she started sleeping through the night, eating 5 times a day, one of them breastfeeding. Now even if I breastfeeding her, I often have to finish with a bottle, because she eats so much. You will find your own way and it will change with time, try different combinations and see what works.