Where to measure runout on VLDs?

So I have a question: When a cartridge is inserted into the chamber without the bullet being jammed then it is 'loose'. Not a rat turd in a violin case but it isnt precisely form fit to the chamber. It has some headspace setback and also some clearance between the body and the chamber walls as well as in the throat.
So in essence is that cartridge 'laying on the bottom' of the chamber when it chambered before firing? If so then what happens to the bullet upon firing if it isnt precisely centered with the bore?! It seems like chamber fit could have as much or more effect on accuracy as the straightness of the cartridge.
I envision on firing the bullet hitting the top of the bore before straightening out.

The recessed bolt face holds and centers the rear of the case. Meaning the cartridge is not laying in the bottom of the chamber.



The bullet is centered in the chambers throat, and the only part of the full length resized case touching the chamber is the case shoulder. And the cone shaped shoulder also helps center the cartridge in the chamber, when the case is pushed forward by the ejector.



And below Mr. Salazar a National competitive shooter is describing how the full length resized case is held and fits in the chamber.

Reloading: Partial Neck Sizing
by German A. Salazar
http://riflemansjournal.blogspot.com/2010/06/reloading-partial-neck-sizing.html

"Now the last scenario, a full-length sized case in which the neck is also fully sized. There is clearance at the neck and in the body of the case, the closest fit anywhere is the bullet in the throat. If the neck to bullet concentricity is good (although it needn't be perfect), then the bullet will find good alignment in the throat and the case body and neck will have minimal influence. Let's not forget that the base of the case is supported by the bolt face or the extractor to a certain degree as well; this is yet another influence on alignment. As you can see, there are several points from base to bullet that can have an effect. My procedure is to minimize the influence of those that I can control, namely the case body and neck, and let the alignment be dictated by the fit of the bullet in the throat and to some extent by the bolt's support of the base. Barring a seriously out of square case head, I don't think the bolt can have a negative effect on alignment, only a slightly positive effect from minimizing "case droop" in the chamber. Given that a resized case will usually have a maximum of 0.001" diametrical clearance at the web, this isn't much of a factor anyway."

ah I didnt think about the spring pressure of the ejector button creating tension to center the casing with the shoulder. thx!