The issue here is using Screen.Width/Screen.Height, which is the current window resolution, not the resolution of the main color target. Instead, use either UniversalCameraData.cameraTargetDescriptor to get the color target resolution. See e.g https://docs.unity3d.com/6000.3/Documentation/Manual/urp/render-graph-write-render-pass.html
Alternatively, use GetTextureDesc on the active color target (https://docs.unity3d.com/Packages/com.unity.render-pipelines.core@15.0/api/UnityEngine.Experimental.Rendering.RenderGraphModule.RenderGraph.GetTextureDesc.html)