Is my baby not eating enough?

I usually feed her every 3-4 hrs like the drs told me to. She has a little trouble eating bc shes a preemie, but she only eats around 30-35 mls. Sometimes she'll only eat about 20-25. I've only had her home since yesterday and I'm worried shes not eating like she should. I dont wanna take her back to the hospital :/ they told me she needs to be eating about 40 ml or finish the whole 2 oz. Shes teeny tiny though so I know her belly isnt big enough to eat that much, and if i force her she spits up.. so I only let her eat as much as she can take.. shes 9 days old if that matters any.