"Regarding the "$64,000 question" what about private sector business/management models that may recommend a ratio of middle managers to employees based upon the industry or organization, its strategic goals and mission?"
Last night's Daily Show had much to say about the number of vice-presidents at Goldman Sachs, but we'll set that aside.
I think the stability of the organization's mandate and activities will play a big role in that ratio. In past, I've mentioned a New Zealand report that examined the stability of accountability frameworks across federal agencies, as a function of the concreteness of their goals and deliverables. Those agencies with goals and outcomes that were more concrete, and easy to measure, had fairly stable accountability/performance-measurement frameworks, where those whose goals and outcomes were more diffuse and abstract kept changing the way they measured performance seemingly every single year.
To the extent that middle managers are not only a means to supervise subordinates, but a mechanism to keep upper management informed, I think that if the agency or agency branch is doing pretty much the same thing the same way, year in year out, it will be an easier task to identify the "right balance" of managerial oversight at different levels and across the agency/branch. As well, the more stable the staff over time (less turnover and internal churn), the easier and more comfortable it becomes to allow staff autonomy, and insist on less oversight. If it is an organization whose purpose is either ill-defined, or constantly being redefined (an organization with an identity problem, and sufficiently unsure of itself as to keep staff on short tethers), chances are that magic number/ratio will be much harder to peg down. It is easy to imagine segments of both the private and public sector where each of those ends of the continuum are true.
But, to reiterate your point, let's hear from some of the HR folks.