Use StaticResource
i.e:
<AbsoluteLayout>
<StackLayout AbsoluteLayout.LayoutBounds="{StaticResource ContainerPosition}" AbsoluteLayout.LayoutFlags="None">
<BoxView AbsoluteLayout.LayoutBounds="{StaticResource BoxPosition}" Color="Maroon"/>
</StackLayout>
<AbsoluteLayout.Resources>
<ResourceDictionary>
<OnIdiom x:Key="BoxPosition" x:TypeArguments="Rectangle" Phone="82,26,60,46" Tablet="111,0,60,46"/>
<OnPlatform x:Key="ContainerPosition" x:TypeArguments="Rectangle" iOS="100,100,100,100" Android="100,80,100,110" Windows="100,120,100,100"/>
</ResourceDictionary>
</AbsoluteLayout.Resources>
</AbsoluteLayout>