Getting the screenspace of the current fragment

I'm tearing out my hair with this, Its close but not perfect

Code (CSharp):

1. void surf (Input IN, inout SurfaceOutputStandard o)
2. {
3.           float2 pos2D = (IN.screenPos.xy / IN.screenPos.w);
4. }

This should work, right, where pos2D = 0,0 bottom left 0.5,0.5 in the middle 1,1 = top right
thus if I draw various meshes with pos2D as red/green it should result in a smooth gradient, but it doesn't I can see the polygons.
Most frustrating Anyone with any ideas whats stuffing up

ignore the shadows in the following but it illustrates whats happening