The best way to close up the case mouth on straight wall cartridges is with a taper crimp die. The Lee FCD is not highly regarded around here when it is applied to pistol cartridges. The rifle version is somewhat more useful.
If your sizing die is adjusted properly, there is nothing for the body of the Lee FCD to do and whatever crimp it may apply is not necessary.
Just close up the case mouth and don't dent the bullet in any way.
"No matter how cynical you become, it's never enough to keep up." - Lily Tomlin