Crystal Reports Add field up to a maximum amount-running total

Message

   I am after the Qty Purch upto Inv column to display qty purchased up to the ending inv value so I can run inventory costing based on actual quantity purchased.
Like this:
QTY PURCH   Running TOtal    Ending Inv     Qty Purch upto Inv
80                           80                  200                80
20                        100                                         20
50                        150                                         50 
60                        210 (should read 200 max)    50 
100                         0                                            0  
running total and Qty purch upto inv should be 0 thereafter because the previous record already met the maximum amount of the field or 200 