I have a pretty screwed up back from genetics and poor choices of my 20's and I have found my max pack weight can be around 24lbs before I start having issues. To keep within this max, I was considering working backwards starting with 2 liters of water (4.4lbs) and 5 days of food (I'm cold soaking mostly and this would be a max load out) subtracting everything from that 24lbs. Is this insane or is this a thing?

How I reached my max number: In short I travel quite a bit and use a backpack rather than a suitcase for convenience. Now granted it doesn't have a hip belt but I have been weighing my backpack for every trip and found what was comfortable or not carrying it through airports, stairs and other such things.