The point most people miss is that the barrels do not converge 'straight'.

The breech-ends of the barrels must be parallel, so that the chambers will be perpendicular to the breech-face. At some point forward of the chambers (usually forward of the forend loop), the barrels are bowed inwards slightly. The amount of convergence required for regulation depends on only the last inch of each barrel.

Placing a laser bore-sighter in the chambers tells you nothing. Placing them in the muzzles might give you some idea.

But remember, each pair of barrels on different rifles will be bowed together in a different curve, which is why the industry has used 'trial and error' regulation for over a century, rather than the impossible empirical solution you are seeking here.

Regulating a DR is usually a challenge, often frustrating, but always immensly satisfying. Just do it!

And BTW, I do have Ellis Brown's little 'Rule of Thumb' card sitting on my bookshelf...

...as a conversation piece!