I must be doing something really wrong. The blue debug text on the image shown was the current reported screen space mouse coordinates. Moving the cursor across the 500x500 grid did not show an change of 500px in either direction. I checked the MyGeometry in debug mode and it does show its size as 500x501 (im guessing 501 is due to padding).
I’ve also checked for scaling with FSlateApplication::Get().GetApplicationScale() however it returns 1.0f, so either that is the incorrect function for scaling/DPI or no scaling is applied.
Any help would be greatly appreciated!