You're not doing anything wrong except thinking that there is an actual, exact answer to your question.
First, you have to realize that the shoulder of the comparator case is probably not the same as the shoulder of your cases. What the comparator is measuring is the distance from the shoulder of the case to the ogive when it touches the rifling. THAT is the distance you need to load your rounds. Shoulder to ogive, not OAL.
Second, there is no magic number for distance to the rifling for any bullet or all bullets. Some combos like to be touching the rifling, some like some jump.
Third, the location of the cannelure is immaterial unless you intend to crimp, which you don't need to do except POSSIBLY for a semi-auto.
Still happily answering to the call-sign Peetza.
The problem, as you so eloquently put it, is choice.
He is no fool who gives what he can not keep to gain what he can not lose.
-Jim Eliott, paraphrasing Philip Henry.