Wow - some challenge.
Unfortunately, most of the 'acuity systems' are based on proprietary information, so users cannot share any information. Short term, I would recommend that you get your hands on ChrysMarie Suby's annual survey of nursing hours. You can order it through her website http://220.127.116.11/LMI/
She also publishes quarterly newsletters.
Chrys conducts annual (voluntary) surveys of nurse staffing - her annual reports are widely used as benchmarks. She also has quantified what she calls "ATD' (admission, transfer, discharge) workload that adds a lot to any department, but is seldom captured in any staffing system.
Unless you organization is commited to purchasing an acuity system, you may have to just go with a reasonable benchmark and begin to carefully -track your case mix index. (Even the bean counters understand CMI) Then you can tie your HPPD to your CMI.
I would also advise you to think about allocating resources based on 2 dimensions...
: the amount of physical labor involved in patient care
: the type of work that only RN/licensed can do - closely tied to CMI
You will likely find that your SNF-type patients require more 'work', but it can be accomplished by NAs rather than nurses.