I need a function to help me calculate my monthly sales. Half of spreadsheet
data is previous months sales/expenses. I want to calculate the one month's
totals based on the previous months totals. In effect what I will be doing
is changing sales totals each month. For example, if October sales were
15,000 (ten months ended Oct) nd I change ten months ended November sales to
20,000, actual November sales would be 5,000.

If there is an easy solution, I would appreciate any input. Thanks.