Wow! Your ratios are a lot nicer than ours. Of course, we're almost always short staffed, but an easy assignment is a 2:1.
On our unit (Level 3 without surgery), we typicaly get
2(2:1) --> that's 2 vents, not always stable
(2:1) + 1 --> 3 babies, 1 on vent, not necesarily stable
HFOV is not always considered a 1:1 either. Our Assistant Head Nurse has decided that they can be paired with 1 or 2 other babies, depending on stability. If the baby is not a yoyo with his oxygen, he is considered stable, regardless of the million drips he might have or the gazillion bloods ordered.
4:1 or 5:1 is only for our step down, where the babies are just feed & grow.
Needless to say, a lot of us don't take a break or lunch