We staff by acuity. Fortunately the unit I work in is well staffed and our assignments are usually staffed the way they should be.
We have "acuity sheets" that each baby has in their chart. We score them based on a number of things (type of vent, unstable or stable, number of lines, IVs, number of meds, labs/gases, ECMO, INO, post surgical, drips, feeds, teaching for parents, etc, etc.) Whatever their score is will give an idea of what kind of assigment to put them in (3:1, 2:1, or 1:1).
Of course it doesn't always work out perfectly, as there are admits, transports, stable kids that get sick, etc. In those cases we just do the best we can and help each other out.