Proper method for setting childAspectRatio in flutter

What is the proper way of calculating childAspectRatio of SliverGridDelegateWithFixedCrossAxisCount in flutter. How to manage the the correct height of each view that is compatible with all devices and should work in landscape and portrait

GridView.builder(
    physics: BouncingScrollPhysics(),
    padding: const EdgeInsets.all(4.0),
    itemCount: snapshot.data.results.length,
    gridDelegate: new SliverGridDelegateWithFixedCrossAxisCount(
      crossAxisCount: itemCount,
      childAspectRatio: 0.501,
      mainAxisSpacing: 4.0,
      crossAxisSpacing: 4.0,
    ),
    itemBuilder: (BuildContext context, int index) {
      return GridTile(
          child: _buildGridItem(context, snapshot.data.results[index]));
    });

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