Dyna - 06 and Up Squish Plate
06 & Up Dyna Squish Plate
When you take the tail light off, there would be a square opening in the fender where the tail light mounts. When looking from the back of the bike, if you use the upper edge of the hole as the line to cut to, with the vertical cut at this point, extending straight down to the ground. This would put the fender just over the main section of the squish plate with the rubber bumper and sacrificial plate extending out from it.
You can also remove the fender and set the squish plate inside and mark it this way as well, by lining up the mounting holes in the fender with those on the squish plate assembly.