I'm not sure I can give a correct worked example, but I'll have a stab (Very old legacy versions of XPZ did go into more detail about how to calculate the diminishing returns, but it took up a bunch of articles for what was essentially just trivia).
Durathread and shawl getting hit for 50 cold damage:
- 1.7 starting cold resistance
- 0.5 boundary resistance
- 0.75 effectivity resistance
Delta boundary resistance
=1.7-0.5
=1.2
Multiply by effectiveness factor
=1.2*0.75
=0.9
Absolute effectiveness
=1.2-0.9
=0.3
Total resistance change
=1.7-0.3
=1.4
Therefore damage incurred
=50*1.4
=70 (so 15 less damage than if you were not wearing a shawl)
Or something like that. I only know of one person who has memorised the full process apart from Dioxine. Everyone else just play by a few maxims:
- If the boundary resistance is better than the armour resistance, then it can be useful
- If you have spare capacity and you really don't know what to carry, then you can consider protective utility items
- If the paperdoll shows naked skin, you'll probably have a sad time in the mountains/arctic
- If the paperdoll doesn't show naked skin, you'll probably have a sad time in the jungle/desert
- If you find a way to wear multiple hats/vests/leggings, don't expect them to stack
- If you think you know how the RF calculation really works, you're probably mistaken
- Hail gasmasks