title: Wpf DataGrid - Adding new rows only when I want them
body: <p>I have a datagrid with two columns a and b. Column b represents a percentage.</p>
  
  <p>I want to have a single editable row at the start, and have them fill in column a and b.</p>
  
  <p>In the case that column b is less than 100%, I want to add a new editable row. I want to always add a new row when the total percentage is less than 100%.</p>
  
  <p>Any ideas on how to do this (using MVVM)?</p>
  
  <p>The datagrid is bound to an ObservableCollection.</p>