Papajoe222 - some of the reasons for using CBTO measurements are that 1) the point on the bullet ogive just above the bearing surface is the point at which the bullet first contacts the rifling, 2) the distance between this point and the base of the case can be measured using calipers/insert with very good accuracy and precision, 3) It is this relationship between the lands and the first contact point on the bullet that we are trying accurately measure, not the overall length of a loaded round (COAL). If a Lot# of bullets has non-uniform nose length (which they pretty much all do), and you use COAL measurements in lieu of CBTO as the determinant for seating depth, it is likely that your total seating depth variance will be very close to bullet nose length variance within that Lot# of bullets, which may be quite large and will generally be unacceptable. In other words, if you have two measurement points (i.e. CBTO and COAL), and the bullet nose region in between those two points has significant length variance, using the longer of those two measurement points (i.e. COAL) will add that length variance into the the shorter of the two measurement points (i.e. CBTO). You really don't want to do that, and there is really no need to do it.
As noted, any additional dimensions or steps added to direct measurement of CBTO is likely to introduce additional error into those measurements, something that is not desirable. There is certainly bullet nose length variance in any Lot# of bullets. However, minor variance in bullet nose length past the critical point where the bullet first contacts the lands will not usually have a huge impact on precision. If minor length variance in COAL still causes "mental" concern or unease, you can always sort bullets into length groups first using bullet OAL. This should greatly minimize the COAL variance of loaded rounds having uniform seating depth as determined using CBTO measurement.
In addition, how you set up your press and seating die can either work in your favor, or against you. Make it work for you. For example, let's say you have decided on using a specific optimal seating depth of 2.250" (i.e. using CBTO measurement) based on your seating depth test targets. You have further decided that anywhere from 2.250" to 2.249" will be an acceptable seating range (i.e. 0.001" total range). Then set the die micrometer such that most of the seated bullets will be at exactly 2.250" after a single stroke of the press. A few may be just a tick longer at 2.251" or 2.252", but none should be shorter than your desired lower limit of 2.249". If necessary, the rounds that measure 2.251" or 2.252" can always be given another light tap in the press to bring them to your final acceptable range of 2.250" to 2.249". The main thing is that you don't want any to be shorter than your lower acceptable limit. By setting up the die micrometer to give you only the desired length, or just a tick longer on the first stroke, you can largely prevent that from happening. Some proportion of your seated rounds will require an additional tap to get them into the acceptable seating depth window, but that is really a minor price to pay for consistent seating depth.
The reason I make that last statement is that it is not at all uncommon to do conduct a seating depth test in .003" increments (some use even smaller increments). It is also not uncommon to see several consecutive seating depth test increments that do NOT produce acceptable groups, then all of a sudden the groups tighten up to a single ragged hole for at least two or more increments. What that really means is that if the bullets are seated anywhere close to the edge of the optimal seating depth window, even a couple or three thousands error would be enough to throw the load out of tune and cause unacceptable precision. When it comes to good precision, seating depth is a BIG DEAL, meaning it's very critical to get it right. Use CBTO measurements, length sort bullets if you feel it's necessary, and make certain that the seating depth of your loaded rounds is consistent.
