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Personally, I would never regulate the barrels so they demonstrate such vertical point of impact, as you mentioned, when making my conversions, from shotgun to double rifle, as explained in W. Ellis Brown's book on this subject (I took class from him, on this subject, prior to his writing his book, now in 2nd edition). Such condition calls for using a "special tool," shown in Brown's book, whereby one tweaks barrel alignment more on a horizontal plane, less on vertical plane, prior to soldering them during regulation. I too have noticed the vertical plane shown on numerous regulation targets, but also noticed a much more desirable horizontal plane with many targets too, so when I chose my three Sabatti rifles, a .450/.400, a .470, and a .500, I tried to find the best regulation target I could, but was not terribly concerned, as I knew that if it didn't regulate well, I could re-regulate it. However, I am quite happy with them, especially the .470 that shoots very well, as good as my better shooting .375 Westley Richards. Additionally, regarding the vertical point of impact regulation targets seen on some Sabatti rifles, when the owners of those rifles shot them, they found much better regulation than their original regulation targets would suggest. I have no idea why that occurred that way. I may be wrong, but I am not yet convinced that firing only two rounds, one from each barrel, for regulation, is enough to show a true state of affairs.
Weight/build of shooters has little or no effect on regulation; at least not enough to become a problem or a concern. Something that has much more effect, is the proper or not proper fit of stock to shooter, manner of holding the forend, resting it on surface or not, etc. I believe that the double, when fired, recoils back; with barrels moving back and upward, to the left, with left barrel, and back and upward to the right, with the right barrel, and the weight/build of shooter is moved with that recoil regardless of weight/build. the heavier shooter will receive the most abuse to shoulder, to tissue, etc., because of the fact that he doesn't move as easily back as the lighter weight shooter, taking more shoulder abuse before moving with recoil.