A formula to accommodate a few simple math conditions

Hello,

I am attempting to find the reimbursement % for a very large data range. The rows are setup so that the first is the charge and the row or rows beneath it (no more then two) are the payments. I am looking for a formula that can accommodate a few different conditions that will occur throughout the data range, which will enable me to drag the formula down. As of now, because I am not able to come up with the appropriate formula, I am inputting the formula in for each set of charge/payment(s) manually. ick! I have an example workbook which thoroughly explains what I need! is there anyone who can help?