DataSource Creation - Request for Ideas

Question

Hi all,
I need some help with figuring out how to do the following. Your help is greatly appreciated. I've simplified the situation, but this will be a great start.
I have 2 custom tables
1) S111 which has fields
    Month
    Mat No
    Goods Receipts
2) S112 which has fields
    Mat No
    Current Stock Qty
I need to create a datasource that has the following fields
-Month
-Mat No
-Goods Receipts
-Stock Qty -- This field stock qty needs to be calculated using a formula (Stock Qty = S112.Current Stock Qty - S111.Goods Receipts)
Questions
1) How should I create the datasource?
2) Should I create a view?
3) Looks like I need a new table/view with the four fields. Where should I add the new field? Where can I write the formula for calculating Stock Qty?
If you can give a general idea as to how to proceed with this, that'll be great.
Thank you,
Anita S.

Answer 1

Thanks for your responses. So, I will create a view with all the fields except the one that needs to be calculated.
Let's say I have data for four months and Stock Qty is initially empty and needs to be calculated.
Please see the logic for populating the "Stock Qty Field". I need to start with the latest month (May) and then calculate the rest of the months
Month===Mat# ==== GR===Current Stck Qty----
Stock Qty (historical - needs to be calculated)   
Jan08===Mat1001===1=== 150----
= (137 - 8) = 129    (logic: Stock Qty = Next Months Stock Qty - Next Months GR) 
Feb08===Mat1001=== 8===150----
= (143 - 6) = 137   (logic: Stock Qty = Next Months Stock Qty - Next Months GR)  
Mar08===Mat1001=== 6===150----
= (147 - 4) = 143   (logic: Stock Qty = Next Months Stock Qty - Next Months GR)          
April08===Mat1001=== 4===150----
= (150- 3) = 147    (logic: Stock Qty = Next Months Stock Qty - Next Months GR)                           
May08===Mat1001===3===150----
=150    (logic: for the latest period, Stock Qty = Current Stck Qty)
Can the logic be implemented in BW or do I need to take care of this in R/3?
Your help is very greatly appreciated.
Thanks,
Anita