The Hornady/Stoneypoint method is very coarse, and you can see with a little research everyone is 'averaging' to pick their numbers. It's flaw is taking datum from both shoulders AND noses.
You already have one variance in bullet nose(for either COAL, or CBTO). Adding shoulder variance separates you that much more from valid measure.
Also, the generalizations about inevitable variances in reloading/shooting -in practice, are also unhelpful or flat out wrong. I assure you it is very possible to set exact CBTO for every single round. I've done it for decades.
There are 2 things to consider in CBTO, as it is nose datum to cartridge base. The nose datum is a variable, but not as bad as imagined out there.
Begin with a soild method for COAL(to a bullet tip); WOODS describes the R-P tool here, which is the cleaning rod method:
Reloader's Nest Forum - OAL
This give you cartridge base to a bullet tip.
Then you take something as simple as it gets to convert that COAL to CBTO:
http://www.sinclairintl.com/reloadi...r-hex-style-bullet-comparators-prod34262.aspx
From here you have CBTO with that particular bullet. Now you can compare that bullet with others in the batch to gauge it as a standard, or not. Of course you might have done this before measuring CBTO.
The only valid way to do this is with a Bob Green Comparator(BGC):
Bob Green New Products
This takes comparative measure of ogive radius from bullet to bullet. While they match your measuring datums hold as same.
If you want your CBTO right on the money, for every round loaded, AND you're using cheap factory bullets, you might as well invest in a BGC. Oh, forget bearing surface comparators, they will not help at all here, because they are including too many OTHER variables.
What if your bullets are better stock, and you're sure bullet noses vary in ogive radius only a small amount? Will it affect CBTO much with a small mismatch? The answer depends on the ogive type, tangent or secant or hybrid. But for the most part and really good bullets your CBTO will only shift up to ~1thou. The reason is that seating plugs take datum on the nose, and luckily this compensates seating well to keep CBTO the same.
A blunter nose leads to higher comparator datum, AND higher seating plug datum, so the bullet will not be seated as deep, and then your CBTO comparator reads close to the same even while higher on the nose. Well, this represents land contact relationship, so it's right as long as measured CBTO is right.
Now imagine adding shoulder datum variances, and I'm sure you'll see this is one too many variables. Stay away from that. Just use simple(actual/known) CBTO that's proven shooting best.
