This was a key question I asked when I shopped for my first Thorn (a Sherpa Mk2). I queried a number of makers, and very few had a ready answer because overall *cargo* capacity depends on rider weight. For example, I weigh 78kg and am 180cm tall, but a friend of mine enjoys touring and himself weighs just north of 175kg and is 200cm tall. As you might expect, this adversely affects the cargo capacity of any bike he rides, and is exacerbated because he requires a much larger frame size.
If you're near average weight for your size, then Thorn's recommended capacities are good ones. However, if you are unusually light or heavy for your size, that will affect how much the bike can carry (because it is also carrying you).
I would suggest weighing the contents of the items you will *actually* carry in the panniers and bags you will use (even empty panniers are surprisingly heavy) so you'll have a good idea of your actual cargo weight. If you have occasional need for extra capacity, be sure to figure that as well. For example, when I do my unsupported solo desert tours for extended periods, I must carry extra food and as much as 26l of water. 1l of water weighs 1kg plus the container, so on those occasions, I carry considerable extra weight compared to my tours in more benign regions.
All the best,
Dan.