Manpower Staffing

I'm trying to make a manpower staffing of our projects. I'm using our ongoing and upcoming projects as activities. Each project/activity has resources like Site Manager, QS, Safety, Carpenter, Steel Fixer, Mason, Helper, etc. Our management wants it shown month wise. For example a project has a 12 months duration, in task usage view I can manually enter the average total of the month using work details with unit of month (mo) like for month of Sep I manually type 5mo for 5 workers. I notice if I move the start date, work values changes. BTW, I'm usnig Fixed Units as Task Type. This procedure is tedious if you have lots of projects and staffs/resources to consider.

I would just ask for advise if anybody there have experience in manpower staffing. Can you please share how you were able to do it, your procedure might be easier.