How to calculate project Man hours and split weekly

Dear all, Please help me if any one can,I calculated man hour as below described Please correct if I am wrong,I used to calculate on thumb rule, For example if the project cost 100.I considered 15% as Company over head,so 100-15=85 aed In which 85 is total cost of labor and material. In that 85,I take 30% that means 25 is my total labor cost then I divide 25 by one hour labor cost, so I get my total man hours, to split weekly I take % of activity on weekly basis and I split the man hours, please Tell me is this wrong or right. Or please suggest me a way to calculate man hours Please Thanks