Alng the same principle and this time you dont need the grid and therefore dont need to know the number of items. Assign all items to the left except the help 🙂
<Menu Height="20" Background="#FFA9D1F4">
<Menu.ItemsPanel>
<ItemsPanelTemplate>
<DockPanel HorizontalAlignment="Stretch"/>
</ItemsPanelTemplate>
</Menu.ItemsPanel>
<MenuItem Header="File">
<MenuItem Header="Exit"/>
</MenuItem>
<MenuItem Header="Edit">
<MenuItem Header="Cut"/>
</MenuItem>
<MenuItem Header="Help" HorizontalAlignment="Right">
<MenuItem Header="About"/>
</MenuItem>
</Menu>