This is what i got but results failed… 
Is this what u ment ?
If i have padding = 0, then my main viewport with no (borders or widgets/render target) i should be able to click the same position on screen as i am in the world right?
Testing:
+I noticed that the results from the equation are almost equal to the GetMousePosition but not exact.
+I Swapped the the rendertarget and viewport around to consider for BEDMAS my bad.