If you mount your engine upright, on top of the rail (red arrow orientation), then the mount should be aligned so that the raised bit pointed to by the red horizontal arrow should be aligned with the firewall "center line". If you mount the engine inverted, on bottom side of the rail (yellow orientation), then the mount should be aligned so that the raised bit pointed to by the yellow horizontal line should be aligned with the firewall "center line".

Yea, yea - clear as mud.