The fourth (crimp) die in the Hornady 454 die set is a taper crimp die. If you are loading bullets that have a cannelure, you should crimp with the seating die, which is a roll crimp. I use this die set for 45 colt, and it works great, but I've never used the taper crimp die. I seat and roll crimp in one step.
You can crimp in the same step while seating, or you can crimp in a separate step after seating by readjusting your seating die.
If you want to seat and crimp in one step (fairly easy to do with roll crimp), follow the die's instructions.
As far as overall length, when roll crimping into a cannelure, you need to adjust the seating depth so that the top of the case ends up in the middle of the cannelure, then make sure it is at least close to the OAL from your reference load.
There is a fairly easy trick with Hornady seating dies to allow separate seating and crimping with the same die, without having to readjust the die each time.
If you want to roll crimp separately, you can set your die up to crimp, by first removing the seating stem. If you pull the spring clip off the bottom of the die, the alignment sleeve and seating plug will fall out the bottom. Remove the plug, but put the alignment sleeve (which has the crimp ring) back in and reinstall the spring clip.
Now adjust your die to roll crimp a cartridge that already has a bullet seated to proper depth. Set the die's lock ring in place and tighten it.
Now get a washer that will fit on the die under the lock ring. Re-install the seating plug in the die, and, with the washer in place under the lock ring, install the die in the press. Adjust your seating stem downward until it contacts the top of a properly seated bullet. Tighten the friction nut on the seating stem.
When you want to seat only, just make sure the washer is in place under the lock ring, or you will seat the bullet too deep (by the thickness of the washer).
When you then want to crimp, remove the plug from the die, take the washer off the die, and it will crimp without touching the bullet.
Andy