How To Get The REAL ViewRect?

I am now trying to make perspective projection effect in slate directly, so I need the ViewRect to do deprojection. While in deprojection I have these to normalize the position :

float NormalizedScreenSpaceX = (FMath::TruncToInt(ScreenPosition.X) - ViewRect.Min.X) / ((float)ViewRect.Width());
		float NormalizedScreenSpaceY = (FMath::TruncToInt(ScreenPosition.Y) - ViewRect.Min.Y) / ((float)ViewRect.Height());

So I need get true ViewRect.Min to ensure calculation is correct. But all the way I get ViewRect.Min is (0,0), while width and height are correct.

I get the ViewRect in these codes( in UWidget.cpp ):

FSceneViewFamilyContext ViewFamily(FSceneViewFamily::ConstructionValues(

		FVector ViewLocation;
		FRotator ViewRotation;
		FSceneView* SceneView = LP->CalcSceneView(&ViewFamily, /*out*/ ViewLocation, /*out*/ ViewRotation, LP->ViewportClient->Viewport);


		FSceneViewProjectionData ProjectionData;
		LP->GetProjectionData(LP->ViewportClient->Viewport, eSSP_FULL, /*out*/ ProjectionData);
		ViewProjectionMatrix = ProjectionData.ComputeViewProjectionMatrix();

		ViewRect = ProjectionData.GetViewRect();
		ViewRect = SceneView->UnconstrainedViewRect;
		ViewRect = SceneView->UnscaledViewRect;

None of these ways get the correct ViewRect
So, the question is How Could I Get the REAL ViewRect which means that ViewRect.Min is the correct position in the scrren such as this:

Where LP is :

ULocalPlayer* LP = GetOwningLocalPlayer();