how to define a grid for Ad control at bottom of page

Hello all,

I'm trying to define a grid that places an ad control at the bottom of the page. I have this:

<Grid VerticalOptions="Fill">
    <Grid.RowDefinitions>
    <RowDefinition Height="Auto"></RowDefinition>
    <RowDefinition Height="*" ></RowDefinition>
    </Grid.RowDefinitions>
    <StackLayout Grid.Row="0">
    </StackLayout>
    <local:AdView Grid.Row="1" >
    </local:AdView>
    </Grid>

I have a custom renderer that adds a control to the AdView of a size of 480x80. I'd expect that it'd move to the bottom and the stack layout would take the remaining space but that's not what i see on windows phone. Is there a better way to do this?

thanks