Credits Assignment to Prior Purchase Invoices

Using Power Pivot, I've built a model that lists prior invoices, newest to oldest, and need to write a DAX formula that scans the purchase quantity and identifies the rows (quantity) that add up to the LinesToCredit quantity.  By way of example, the model results in 13,769 rows with NDC and account number concatanates, purchase events (invoice num) and purchase quantity for each sale.  I need DAX formula that scans the purchase column, identifies the rows that add up to the number quantity in lines to credit (repeated on each purchase row).  In the example below, how do I identify how many rows it takes to sum to the seven (7) LinesToCredit to in Purchase Quantity?  In this example, there are 19 purchase events (rows) totaling 22.  It takes 4 rows to equal the credit quantity of 7.  Once I've identified the rows to 'assign', the remaining rows can be removed.  I have been manually scanning the Purchase Qty rows and making the assignment and want to automate this action.  gan5133