Water is not a problem, I lump this in with foraging. Two full 600ml Gatoraid bottles are all I bring. I have a 3.5ounce bladder I carry IFF I expect a dry camp. The water is added in to pack weight. This is measured by simply weighing it at the trailhead with a digital bike scale, give or take about 10g. In the ADK's, there is more water sources around than a lot of places. No worries.
I have several packs I use for light loads. For the past 10 years or so, I have wanted, then found, real UL packs for this loading. GVP, later Gossamer Gear, made some good packs, and some pretty crappy packs. Anyway, I have been on 10day trips with 23 pounds. 13day trips with 27pounds. 14 day trips with 29pounds. and so on. Being retired, I get to go out for about 60 days a year. I have not carried 30pounds since my last 6week canoe trip over the NFCT, but that was supported, in the sense that there were rest stops no further than 7-10 days apart. G4(17oz,) G5(6.8oz), Miniposa(15oz), Gregory(2#8), Tough Traveler(2#2), Kelty(4#14), Gregory(~5#13?), Osprey(3#7), Golite(2#2), Mountansmith(2#4) are a few I have. I replaced the G5 with a Murmur this year(8.5oz.) I have another half dozen packs around, but these were mostly used once and never used again for one reason or another. The Porter/Expedition would end up among these, I am afraid. Probably make a good guide pack or canoe dry pack, though.
The overall percentage of dead weight to weight carried by a pack is just a rough estimate. Comfort is always the first criteria. With ~20pound loads it doesn't much matter. I can carry almost any pack at that weight, so, I look for a good light pack. Heavier loads require more, generally stiffer frames, so the percentage tends to climb a bit. Manufacturors will curcumvent this by adding a stiffener to an otherwise framless pack. But, stiffeners lack good attachments, soo, this is limiting. This is also due to human physiology. You can carry 20 pounds all day, mostly without noticing it much. 30 pounds starts getting heavy. 40pounds is heavy. 50 pounds is very heavy. Without training, this is about the limit. With training, you can carry up to 100pounds or more, but this *does* require training. But, you can be a hundred pounds overweight and carry that around ALL THE TIME. Conditioning? Training? Whatever. ~5% for 20pound loads is OK, with carefull searching you can go considerably less. An example: a 1/2pound pack for 20pounds gives about 2.5%. this jumps to about 7%-8% by the time you hit 35pounds or around a 2#13 pack. About 10% for 45-55pounds or around a 5#8 pack. This is about the minimum weight needed to support those loads. Lots of variables, and your milage may vary. It jives fairly well with my observed comfort with variuous packs, though. There are lots of manufacturors out there. Some sacrifice comfort to save weight. Some sacrifice weight for comfort. I am sure you can find examples both ways.
Here is a rough graph: