JSL to create a column that is a cumulative sum based on previous row of other columns

[#1]

I am using JMP Pro 12.

I need help to create a new column ('cum_Qty') in JSL that will be an ongoing sum of the previous rows value in 'Qty'column until meet => 500. If the row qty is => 500 in , the value will update as it in the 'cum_Qty'. 

Example:

In workweek 2, the qty is 633 (> 500) in 'Qty'column, this value is updated as 633 in 'çum_Qty' column.

In workweek 3, the qty is 185 (< 500), it sums 185 + 633 = 818, this value is updated in 'çum_Qty' column.

In workweek 4, the qty is 130 (<500), it sums 130 + 185 + 633 = 948, this value is updated in 'çum_Qty' column.

and so on.......Refer attached file for details.

Thank you for your help!

[#2 · Accepted solution]

Here is one of the ways to code this.  It is a column formula

If( Row() == 1, x = 0 );
If( :Qty > 500,
	x = :Qty,
	x = x + :Qty
);
x;

[#3]

Thanks Jim. It works perfectly!