Tracking Sales Projections

Hello everyone and thanks for reading. Brand new to this so please be gentle.

I need to create a worksheet that will track sales projections by customer/div

How do I make a formula that will automatically reduce the "projected sale number" each time an order comes in? Since that may be 1 to 100 orders through the month; is this best to do in a column per customer, rather than row? Also how to I continue the formula down a column since I am not sure how many orders will come through in a month.

Here is what I would like to see but I am fuzzy on how to do it

CUSTOMER NAME/DIV
PROJECTED SALES AMOUNT
REMAINING AMOUNT DUE

DISTRO AMOUNT RCV (minus)** DATE SHIPPED

Thanks in advance... I appreciate the help