Looks like that'll be what I have to do, although I'm not 100% sure how to get a solid measurement other than to 'air measure' this dimension and rely on just eye-balling it to estimate when one side of the mic is in the center of the bore.
I've not taken it the action out of the stock ever (just a few weeks old - the rifle, that is), so I'm not sure if it's symmetrical about the bore axis or not. Otherwise, yeah, I could do what you're saying and easily come up with a measurement.
Open the bolt and measure from the middle of the bolt to the top of the receiver. I'm pretty sure that is all I did. There was a link posted on here awhile ago talking about measuring this if I recall. I could be wrong though.