Krepinevich explains why this was so hard for Americans to do:
Attrition is a product of the American way of war: spend lavishly on munitions, materiel, and technology to save lives... U.S. military leaders believed in the morale-raising and life-saving value of massive firepower whose success they had witnessed in World War II and Korea. Krepinevich goes on to show how the American strategy and tactics were completely inappropriate for a real counter-insurgent strategy, but instead were designed and implemented in such a way to achieve a maximum enemy bodycount. He also depicts America's force mixture as inappropriate for the type of war being waged. America's 'foxhole strength' of 80,000 out of a total force of 550,000, Krepinevich believes, illustrates an inefficient force mix (176, 197).
This is quite similar to the notion of a tooth-to-tail ratio, or the ratio of combatants to total forces. Most pundits would agree that a 1:6.88 ratio is poor. For Krepinevich's mission of protecting the population from the insurgents, the more ground-pounders the better. For Nagl's mission of turning the population, teeth per tail is a meaningless ratio.
I would like to propose an alternate measure: the ratio of people assisting the locals (whose hearts we seek to win) to the number assisting our own soldiers. Soldiers who are actively working to up the friendly count would include doctors operating on locals, engineers building schools, etc. Soldiers in the second category would include those protecting American soldiers, those providing services to American servicemembers, etc. The numbers will be somewhat imprecise because of difficulty determining if a soldier providing security is securing them or us. Some imprecision, however, does not reduce its relevance.
Nagl speaks to the importance of the economical use of manpower (40). Additionally, in a situation like Vietnam (or Iraq), the smaller footprint the less likely it is to alienate the locals. Therefore the more positive benefits reaped by the local population (represented by the number of troops actively engaged in helping them) per the size of the American footprint (represented here by the others), the more efficient our force.
Tooth-to-tail ratios may be relevant to attrition warfare, and possibly even to an oil-spot strategy, but it isn't helpful to analyze the efficiency of a force with a hearts-and-minds mission. Maybe this Helpers-per-footprint approach will be more clear.