Update a field based on line item update

I am trying to make it to where my script pulls quantity every time a person adds a new quantity on the line item. I currently have this on a postSource function:

var itemQty = nlapiGetCurrentLineItemValue('item','quantity');
var extDollar = dollarPro * itemQty;
nlapiSetCurrentLineItemValue('item','custcolextdollarprofit',extDollar);


which only populates my custom field (custcolextdollarprofit) correctly if I add the quantity before I add the part on a sales order. If I try to change the quantity after I add a line it wont update. How can I make it repull the quantity so my calculation updates?