If they are struggling with how much effort it requires or get bored on the bottle. You can always try and go back if the flow is too fast.
We used 0 and 1 interchangeably as didn't see much difference in the amount of effort or any reflux/signs that the flow was too fast.
Thank you both 
If they are struggling with how much effort it requires or get bored on the bottle. You can always try and go back if the flow is too fast.