To define the percentage in the width, in the example below, instead of 70%, replace it with .7*.
<Grid>
<Grid.ColumnDefinitions>
<ColumnDefinition Width=".7*" />
<ColumnDefinition Width=".3*" />
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*" />
</Grid.RowDefinitions>
<TextBlock Grid.Column="0">70% width</TextBlock>
<TextBlock Grid.Column="1">30% width</TextBlock>
</Grid>
