Calculating cells values

Hi Guys I have 2 datatables
dtInvoices & dtInvoiceDetails

dtInvoice has the following columns
date - orderNo - net - vat -Total
I have 2 methods of editing dtInvoiceDetails inline & the editor window.
In this table I have columns
quant - sell - nett - lineTotal - vatRate

What is the event so I can calculate the lineTotal when the quant/ sell amounts change ( either by inline or the editor window)

I also have need the sum of lineTotal to add to the dtInvoice table.

I have this so far.

On the dtInvoice click

  $('#dtInvoices').on( 'click', 'tbody td', function () {
        //alert( tableInvoices.cell( this ).data() )
        currentSelectedInvoiceIndex = tableInvoices.row( this ).index() ;
        alert(currentSelectedInvoiceIndex);
} );

The invoice total is column 2 so I can set the sum to cell(currentSelectedInvoiceIndex,2)

I can't find an example showing this.

So I need

1. Get the onChange event of a row/cell and calculate the line total
2. Then calculate the sum of the column lineTotal
3. Set the total in dtInvoices.

Cheers

Steve Warby