our staffing is based on how "toxic" our unit is for the day. of course we have the official on duty nurses for the day, but if more patients are being admitted someone definitely has to do overtime, or someone who has the day off could be called in. just a day in the life of a nicu nurse.
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.
We staff by acuity. We don't use any formal system of grading (maybe we should; it would probably have saved me from getting stuck last night with a 24-hour-old micropreemie plus "sharing" a 3-month-old, who refuses to eat, with my preemie's twin's nurse), but we always have adequate staffing for what the acuity level dictates. See, there are (some) perks to living in California!
I think our HPPD used to be 13, and just got bumped to 19-20 this fiscal year. It opened up an additional 25 RN positions. We are almost fully staffed, and assignments have been VERY doable recently, except for the 81-baby census blip we had a few weeks ago. Our per diems and OT-hungry staff loved that, and we still remained fully staffed.