Position widget in stack with percent

You can combine a Positioned.fill and LayoutBuilder to achieve such result.

    new Stack(
    children: <Widget>[
      new Positioned.fill(
        child: new LayoutBuilder(
          builder: (context, constraints) {
            return new Padding(
              padding: new EdgeInsets.only(top: constraints.biggest.height * .59, bottom: constraints.biggest.height * .31),
              child: new Text("toto", textAlign: TextAlign.center,),
            );
          },
        ),
      )
    ],
  ),

one thing that i figured out not long ago is that you can position a widget on the screen using a container its alignment parameter with the help of the Alignment.lerp(x,y,z) function

//the widget will be placed in the center of the container
alignment: Alignment.lerp(Alignment.topCenter, Alignment.bottomCenter, 0), 

//the widget will be placed in the bottom of the container
alignment: Alignment.lerp(Alignment.topCenter, Alignment.bottomCenter, 1), 

//the widget will be placed in the bottom quarter of the container
alignment: Alignment.lerp(Alignment.topCenter, Alignment.bottomCenter, 0.5), 

//the widget will be placed in the top quarter of the container
alignment: Alignment.lerp(Alignment.topCenter, Alignment.bottomCenter, -0.5),