Derive Region of customer in Sales order

Hi,

I want to derive Sales order revenue based on region of customer which is in sales order.

Means in a sales order if customer is belongs to one region based on that it needs to derive particular region in COPA. Because we have 12 regions and under each region there are different customers. in each customer we update region filed also (REGIO)

So here i want to use Table-Lookup (derivation method) to derive the value from sales order in that region.

Kindly tell me how can we achieve this in table lookup, means which table i need to select and which which fields i need to choose.

Please let me know if any details require.

Thanks in adv,

Ashok