It isn't that simple, and yet simpler still.
Throats are cut at whatever angle the reamer provides. It could be from ~.5 to 15degs.
Bullet noses come in various forms(tangent, secant, hybrid) and radius ~8 to 15degs.
Various combinations change the contact points of both lands AND noses.
You can order your reamer with a throat angle that matches a particular bullet nose.
You can have a comparator made with the barrel stub and same throating from that reamer at barrel finishing. Only this would provide for true contact datum from a comparator.
Change bullets and you negate the efforts.
But it doesn't matter, as a relative datum is typically what we define while discovering most accurate seating distance -with any particular bullet. You can log it, and as long as you always use the same tool & method of measure, then you can reproduce that best seating distance.
I don't pay any mind to where a Sinclair nut contacts w/resp to loaded contact. I'm satisfied as it get's that I can reproduce with the nut whatever I measured with it as best seating.
Barrels were origionally bored and then rifled. Bore diameter is, by definition, the size hole that gets precisely bored during manufactoring, groove diameter is to the bottom of the rifling. Ergo, a .30 cal barrel is bored to .300" and, with a normal rifling depth of 4 thou, the groove diameter is .308".
Seating a precise amount off the lands isn't the goal, finding the best shooting jump to the lands is but that's a relitive thing,it matters not if we know to the last thou what it really is.
A seating variation of +/- 5 thou rarely matters unless we're seating on a ragged edge of what works well..