Crystal Reports How to calculate qty used ?

Message

   Hello folks,

I have read and searched the forums but to no avail!

I have tried to create a formula to calculate the qty used against a qty recieved. 

my data looks like this

date                      type          qty       value £              qty used
01/01/2008          ADJ IN        10         £1.00                    10
02/01/2008          ADJ OUT      -5         £1.00            
05/02/2008          ADJ OUT      -3         £1.00
10/02/2008          ADJ IN         15        £1.25                    1
15/02/2008          ADJ OUT      -2         £1.00
22/02/2008          ADJ OUT      -1         £1.25       

So far I have grouped the data fine, no problem. Si have a list of transactions between two dates.

However I cannot find a method to set the qty used for an ADJ IN to equal the ADJ's OUT. 

i.e. if type = ADJ IN then qty used  = ADJ OUT up to the value of the ADJ IN, then move to the next ADJ IN.

I have been banging my head for the past two days, so if anyone has any ideas it would be greatly appreciated!!! 

Thanks in Advance,

Amblin