Stamina decreasing process is following:
[Stamina decreasing] = (Damage / Before Life) * (Before Stamina) - {(Reduction of stamina decreasing) - 5}
Reduction of stamina decreasing is determined by the composition of the armor.
You can have following points from per part of armor.
Metallic armor:7.0
other non medable armor:3.5
medable armor:2.0
Your reduction of stamina decreasing is sum of large five parts.
Human has extra reduction from race bonus.
I'd done some testing to arrive at this approximate equation. For instance I knew that the higher your HP when you get hit, the less stam you lose for hits of the same value. I always assumed that it was relative to your own max HP, not just the raw number - though admittedly I don't have any characters with, say, 80 hp laying around to test what getting hit at 55/80 is like vs 105/150. I also didn't know that the higher your starting stam, in raw form, the more you lose; but again, I never thought to try with some char trying to melee with 95 stam.
I ran some numbers.
Once you start losing stam, you'll lose 1 or 2 more per 1 damage taken. For hits of the same damage amount, you'll lose 1 or 2 more stamina for every 2 or 3 less starting HP you have. As you see, it works in the opposite direction for stam when you get hit: for every 4 or 5 less you have, you take 1 or 2 more damage before you start losing stam.
If you wear 5x metal, have 150 HP when you get hit, and 160 stam when you get hit, you won't lose any stam until you take 34 damage.
If you wear 5x metal, have 150 HP when you get hit, and 190 stam when you get hit, you won't lose any stam until you take 29 damage.
If you wear 5x metal, have 150 HP when you get hit, and 215 stam when you get hit, you won't lose any stam until you take 25 damage.
Interesting it'd be worth confirming on T/C maybe, but that seems to tell me that investing in more SSI vs less SSI but more stam might be worth it to run the same weapon...
Some other typical combos:
If you wear 4x studded +1 metal (like a blackthorn's mace/shield), have 150 HP when you get hit, and 190 stam when you get hit, you won't lose any stam until you take 17 damage. (big difference...)
If you wear 4x studded +1 metal (like a blackthorn's mace/shield), have 150 HP when you get hit, and 215 stam when you get hit, you won't lose any stam until you take 15 damage.
If you wear 5x studded, have 150 HP when you get hit, and 190 stam when you get hit, you won't lose any stam until you take 14 damage.
If you wear 5x studded, have 150 HP when you get hit, and 215 stam when you get hit, you won't lose any stam until you take 13 damage.
If you wear 5x meddable (say woodland maybe), have 150 HP when you get hit, and 190 stam when you get hit, you won't lose any stam until you take 9 damage.
If you wear 5x meddable (say woodland maybe), have 150 HP when you get hit, and 215 stam when you get hit, you won't lose any stam until you take 8 damage.
My particular setup: 3x metal, 2x studded. 150HP, 194 stam. I lose stam from 21+ damage taken. Anecdotally, this seems spot on with what I've observed.
