Excel functions spread over several columns?
 

I've got a spreadsheet that I enter sales values into every month. Each month
has 2 columns, one for sales and one w/a formula to calculate points accrued
from the sales for the month. This spreadsheet has a 5-yr history, and this
year, we are dropping points that are 1 year old. When the points are spent,
a negative value goes into the formula column so that when the total point
value is seen, the current total is seen (total minus what was spent). I need
to be able find what will drop off every month (in September, last August's
accrued points will go away), BUT - I have to figure in the total of what was
spent. IF they've spent the points from the past, they won't be losing any...
10,000 pts over the last yr, say equal amt/month. Points were spent in March
and August, not all of them, way less than half. How much will we lose in
September?
I'll add more details if necessary.