I have found that "best" taper crimp is when I can not see any case mouth flare when looking at the round in front of a well lit white background and when I can not feel any flare when I run my finger down the bullet ogive to the case. This will almost always leave an unmarked bullet.
I agree 100% and feel this needs to be empathized. It's not technically a "crimp" at all, but really just the straightening out of your case mouth after you flared it to accept the bullet. That's it, that's all. Many bullet seating dies will do this operation for you even if the die set comes with a separate crimp die.
And the goal isn't trying to make your reloaded rounds "look" the same as factory cartridges. OAL should be measured by using your gun barrel as a case gauge. Just short enough to fit into the chamber and drop out easily is good, plus a little bit more for a margin of safety.
Most pressure problems in 9mm occur when the tension is too light holding the bullet, and bullet setback occurs during chambering. Tension can get too light if you over crimp. Copper is more elastic than lead when reshaped. Just keep that in mind. That characteristic can either be your friend or foe.