Getting the coordinates of a rectangle on the screen has always been a little awkward for me. I usually press CMD-SHIFT-4 and then try to read the tiny numbers on the screen. The KM Editor also has a Mouse Display window which can be very helpful, but it's intimidating. I have found a new way to find the location of a rectangle on the screen. Just open the KM Editor's Value Inspector window and enter the following item in it:
Then move and resize the Value Inspector window until the area you want to select is covered up by the window, and the four numbers in the window will be the X,Y,W,H coordinates of that area! Simple.
Now it could be even better if the window could be made semi-transparent so you could see through it to confirm that you have it in the right location. Maybe this could be a feature request. This window has a flag to make it "always on top" but it could use a second flag called "translucent" which would make at least the potion of the window under the title bar about 50% translucent. Now that I mention it, the Progress Bar window should also have a translucency setting (at least settable in the action, if not also by the user.)
Also, the Width and Height need to be a minimum of 451,119 (your results may differ depending on your screen resolution.) So this method isn't good for locating "small" rectangle sizes.