Manpower planning, or for that matter any task in HR, has to be a quality job. I have not come across a quantitaive manpower planning unless you mean you want to produce a verbose report which actually means nothing!
A manpower plan has to be linked with the Annual Operating Plan ( annual budget) and has to be in sync with the business plans. The person must have a clear understanding of the business cycle in order to prepare a comprehensive manpower plan. The plan has to be discussed and agreed withe the business Head and the MD of the organisation. It entails expenditure by the organisation to get people on board. One can have a quarterly review of the paln and actuals and may be able to tweak and modify the plan to suit the busiuness environment.