If you want a Margin of say 70%. divide the CP by 1-0.70, i.e.CP/ 0.30. Alternatively make the CP 1, and add 1 to the answer to give you a Mark-up factor to multiply by. Thus 1/1-0.70 would give you 3.33, add 1 to give 4.33 and this is the Mark-up factor that will give you a 70% Margin (i.e. CP times 4.33 will give a 70% Margin) and so on.