Translation of pose to a point 500 mm in front of the AR tag

I'm using AR-tags to automatically detect locations to visit using AMCL navigation. When I receive the ar_pose_marker event I transform it to the odom frame and save this for navigation later on. The challenge is that I cannot navigation to this exact point because the marker is placed on the wall and my robot will then hit the wall. So what I need is to navigate to a point 500mm in front of the AR-tag. But how can I make the translation of the pose?