Anytime you get into definitions of lightweight/heavyweight pack sizes it is fraught with problems. I agree with you Glenn that the OP doesn't take into account BMI, but I would suggest your step 3 overcorrects. I am obese enough that all of my pack weights would be negative using your formula. However, I've been obese and active my whole life. I can carry a lot more weight than a reasonably fit person my height with a healthy BMI. I am used to being active with the weight and it isn't as awkwardly hung on me as a backpack. It's also true that a reasonably fit person with a healthy BMI at my height could probably hike me into the ground with the same size pack. Maybe "half a pound on the waist is the same as a pound on the shoulders"? I don't know. Like I said it gets difficult really quickly.

I think Age, BMI, and % body fat (independent of BMI) all play a significant role along with time spent hiking with that weight on your shoulders.

I do notice the people who get really proud about how low they can get their base weight are short and lithe. Everything they carry weighs considerably less than my stuff even if we have the exact same brand and model. (I am also jealous of how much farther they can hike in a day than I can )