I'm new to Xamarin so excuse me if this is very obvious.
I added a Grid to my ContentPage. I want the grid to occupy the entire ContentPage, which I assume is the full size of the device. In my case I'm testing an iPhone 6S simulator. The first row should be 7.58% of the total height of the Grid and the last one should fill in the rest. I add the label to see where the bottom of the last row is and it shows all the on the top. It looks like it's ~7% from the top which means it's the bottom of the 1st row.
<Grid x:Name="layoutGrid" HorizontalOptions="Fill" VerticalOptions="Fill">
<Grid.RowDefinitions>
<RowDefinition Height=".0758*"/>
<!-- Logo -->
<!-- Remaining Space -->
<RowDefinition Height="*"/>
</Grid.RowDefinitions>
<Grid.ColumnDefinitions>
<ColumnDefinition Width=".154*" />
<ColumnDefinition Width="*" />
<ColumnDefinition Width=".154*" />
</Grid.ColumnDefinitions>
<Image Aspect="Fill" Source="{local:ImageResource App1.Resources.background.png}" Grid.ColumnSpan="3" Grid.RowSpan="11"/>
<Image Aspect="AspectFit" x:Name="settingsImage" Grid.Column="2" Grid.Row="0" Source="{local:ImageResource App1.Resources.settings-17-xxl.png}" HorizontalOptions="End"/>
<Label x:Name="lblTest" Grid.Row="1" Text="Bottom" TextColor="White" VerticalOptions="End" HorizontalOptions="Center" FontSize="18"/>