Whoever came up with that 20% figure must have been a big, strong man!
I've just done some calculations which show that my pack weight is only two thirds of 20% of my body weight (which makes it...um...er...I'll let you work that out).
Even if you added the weight of my clothing and contents of pockets, plus threw in my boots for good measure, and why not my walking poles too, I'm sure it still wouldn't be near that 20% firgure.
Forget about calculations, just concentrate on only taking the essentials, and making those the very lightest you can obtain.
You'll know the difference when you pick your pack up easily and walk off as if it's part of you, rather than struggling to lift it off the ground, staggering as you get your arms into the straps, and then feeling as though your legs are compressing and your feet are sinking an inch into the tarmac as you take your first few steps.