Auto fill formula every nth cell, and look up value every nth cell in source

OK i know my title sounds kinda confusing, but i have a huge excel sheet for which I must produce a result (please see the attached sample).

Source tab contains vital information about some clients.

In the aggregated tab (Cell C10) I created a formula that pulls the Inflows from the source in a very specific array. So for client 1, this works fine. Now, if i copy my formula to the client 2 (Cell C14), it obviously wont go and look in the correct array in my source.

What i need to do is to be able to copy/paste my formula

Formula:

Please Login or Register  to view this content.

(from cells C10 to CC10) to cells C14 to CC14, but when copied, the look up array changes to:

Formula:

Please Login or Register  to view this content.

I will have to fill this formula to at least 100 entries down, so i need to make it work with ease

The good thing is that all look up values in the source increase by a fixed number of rows (12). I tried playing with index/rows formula.. no luck..

any help would be greatly appreciated!

anyone please?