I was surprised that it wasn't working for you. I had shared a demo macro in How to Sort a List of Filenames by a File Property - #3 by August, a macro called "Get Center of Icon, Show Rectangle" which was working just fine for me.
BUT I had my Icon View of my folder set to "Hide Tab Bar" and "Hide Toolbar" (which also hides the Sidebar), so the two points in your diagram above are the same. With that display change, I am easily able to draw a rectangle around the chosen icon.
So is there some way to find out how much space the Toolbar takes in Y, how much the Sidebar takes in X, and the Tab Bar takes in Y, and adjust accordingly?
The "Window Related Tokens" page in the WIki makes it look like the token %WindowFrame% has four values in it, the first two are %WindowPosition% and the second two are %WindowSize%. The implication in the WIki is that those are the same values.
I'm sure that I've seem parameters somewhere that separate the center active area of a window from the toolbar, sidebar, status bar, etc. at the top, bottom, and sides. Is that possibly accessible via AppleScript? I'm not finding that distinction in KBM anywhere. Is it in the OS UI controls or am I thinking of HTML?