X dimension is measured vertically (not along the transom angle) from the center of your hinge pin or trim sender (which is the centerline of your crankshaft) on the side of your gimble bracket to the bottom of the pad.
You'll need to get a straight edge of some type (yardstick/level/framing square/etc) parallel to your pad and sticking out beyond it, and measure straight down to it from the trim sender.
Regards,
Steve