Try changing the add row button to have the following JavaScript
var newRow = Table1._Row1.addInstance()
if (newRow.index > 0)
{
newRow.NumericField1.rawValue = newRow.resolveNode('Row1[-1].NumericField1').rawValue + 10;
}
You will have to change this code to match your form names, but basically the addInstance() method will return the new row, the newRow.resolveNode('Row1[-1]... will get the value of the previous row and then we add 10.
Regards
Bruce