formula question (HIPP mamas)

for the mom that use hipp organic formula. I have a question how many boxes do you buy a week. I looked online and just for a guideline it said 2.5oz per LB of baby. so if 800g of hipp formula is roughly equivalent to 28oz and let's say I have a 10lb baby (my husband was 10lbs and my son was 9lbs) that would mean my daughter would go through roughly 23oz of formula a day. so I would be going through basically a box a day? I'm so confused, I only EBF my son he stopped a few months ago once he turned 1 and now I'm due in 15 days and for personal reasons I will try to BF but I think I'm just gonna formula feed. I only want organic formula for her, and I know there are other organic ones out there but I'm leaning more towards Hipp and Holle, I will do honest if I have to but Hipp is my first choice. I also would like to know about how long did your formula last once opened. and how much should I order if I decided to only give my baby colostrum. I would hate to run out before I got in my next order. thanks in advance!