This is one reason for adopting a
from the skin out weight target than the more commonly discussed pack base weight. I haven't spotted
@t2andreo on the camino, but I have seen walkers who have bragged about how they have managed to get their load into a smaller pack, but have been wearing a huge waist bag and carrying almost as much outside their pack as inside. IIRC, Tom makes no secret of this, and had admitted to the volumes and perhaps weights that his approach includes over the basic pack at some point in the past.
My own view is that there is much well intentioned, but relatively useless advice in this thread. Why? Because the OP is both taller and heavier than the average pilgrim that I see when I am walking. Very few people have either acknowledged that fact or informed us about their own height and weight. Without those things to assist make comparisons, suggesting that the OP might find the same pack as any of us found suitable is rather fanciful.
While we may be cautious about the 10% rule of thumb, it is not unreasonable for the purpose of helping them get to the right size pack to assume that they will carry that much for their summer camino. That is, their pack base weight might be about 8.8 kg. Carrying that in a 34 li pack is a packing density of 258 gm/li. In my experience, that is going to be incredibly tight, and very difficult to achieve without carrying a significant amount of that external to the pack.
Earlier,
@henrythedog admitted to using a 38 li pack loaded with 8 kg, or around 210 gm/li, which I think is still a bit high for comfort, but far more reasonable. My view would be to try and keep the packing density to 200 gm/li or a little less to keep the load within the pack. Reversing this, my recommendation to the OP would be to consider a pack around 45 li.
One could do a more sophisticated calculation by discounting the pack weight from the calculation. Again, using
@henrythedog's figures of 6.5 kg of load in 38 kg pack, this equates to about 171 gm/li excluding the pack weight. My target is a little less, around 160 gm/li. The tradeoff is best illustrated with the two Osprey packs the OP is considering. One weighs 42 gm/li of carrying capacity, the other just 31 gm/li. The Stratos 34 would carry 5.8 kg @ 171 gm/li, and have a base weight of 7.2 kg. The large Exos 38 would carry 7.0 kg @ 171 gm/li, and have a base weight of 8.3 kg. Clearly quite different outcomes.
There has been much written by more experienced forum members about how to select a pack. It is worth searching for, but the nub is always (a) to get your gear together and take it with you, and (b) get help from other forum members if you want help with what to pack.