Calculating a running percentage

Hi folks, could you help me with the attached file please? I'm having problems getting my percentages to calculate. It works the way I have done it (by adding the individual cells to the formula) but it's not very nice and if I add more lines in I'll need to edit the formula. The formulas are in colum H. I know there is a way to do this using ranges but for the life of me I can't remember how to do it.