I don't know all of what you are dealing with.
Is there a problem with loading to the tested book published length?(disregarding cannelure)
The bullet likely was designed for 7.62 X51 . There is no reason to expect the cannelure to be in the "right place" for 300 Blackout.
One factor I consider is seating the bullet base the full depth of the neck length.
The 300 Blackout has a short neck. I'd take full advantage of it.I'd want maximum bullet to cartridge neck contact. If its a boat tail,I'd hang the boattail into the powder space.
It needs to be not over 2.260 for magazine length. Beyond that,most of COL (in this case) would be feeding reliability.
I'd ignore the cannelure. Its irrelevent to 300 Blackout. Unless you have another specific reason not to,I'd load to Hornady's recommended 2,235.
You asked about headspace. Its apparent you are not clear on the meaning of the word. Bullet seating depth has nothing to do with headspace.
I suggest you study the topic in your reloading manual,or use the search function here to dig up past threads. Its a good subject to know about.
Headspace is all in the gun. Not the ammo. Its the distance from the breech face to the chamber feature(shoulder,in 300 BLK) that limits the forward travel of the cartridge in the chamber.
Head Clearance is the term used to describe the clearance from the cartridge case head to the breech face when the cartridge is full depth in the chamber,contacting the chamber feature (chamber shoulder in 300 BLK) that stops the cartridge from going deeper.
Headspace is in the gun,and only the gun. Short of gunsmithing,nothing you are going to do will change headspace.
Head clearance is about how your ammo fits the gun,and that can be affected by your handloading practices. But that is around resizing the cartridge case and sizing the shoulder *
The bullet seating depth is not a headspace item...or it should not be.
* Rimmed cartridges headspace on the rim. Belted cartridges on the belt. Straight wall rimless cartridges on the case mouth.Rimless bottleneck cases use the shoulder.