
Here's something I threw together rq. The script is in the change event for the checkbox in the table. (Of course, you'll have to modify it to suit the names of your fields.)
var rows = xfa.resolveNodes("tblAmounts.Row1[*]");
var subtotal=0;
for (i=0; i<rows.length; i++) if (rows.item(i).cbAdd.rawValue == 1) subtotal = subtotal + rows.item(i).nfAmount.rawValue;
nfSubtotal.rawVlaue=subtotal;