Yes, I know there isn’t a perfect value for the effect that I am going for, but I still need some best values as possible for each corner. (I have been trying to experiment with this effect for half an hour now and what I’m doing isn’t what I am looking for.)
I’m not sure I understand what you mean by “best values”, since every project will be different.
Fo me, the simplest method is to start by manually moving the 4 points in the VUI (preview window), then fine tuning each of them in the parameters (suggestion: use your mouse wheel to edit the corners coordinates).
Then you can maybe add feathering and a Blend Mode filter for more realism.