I was just dealing with this bed calibration issue over the weekend.
My home position on the bed is lower than the back right position. After pulling the bed apart I determined that my heated aluminum plate is not completely flat for one, maybe from colliding with the print head, and the corners have different degrees of height. The middle seems to be lower than the rest.
The biggest culprit though was the rear spring on my UM2. It is badly curved, effectively angling the rear of the bed in whatever direction its sitting in since there is only the one point creating a fulcrum. I rotated my