Nice idea

Maybe as capstone, 20 as full heavy and 10 for light armor

The formula is on your side here. Damage taken is your damage squared over four times your armour, so +10 armour cuts about a third of a hit at 20 armour and under a tenth of it at 100. A percentage is worth the same at every tier.

So Thick-hided is a percentage now: +3% armour a rank, and another 5% when you carve the fifth rank, which is +20% at rank 5. The flat +2 is gone.

Not quite the 5% a rank you asked for, because 25% on top of a full late set is a lot of armour for one stone to hand out.

Steady footing's capstone went the same way, +5% instead of +2, so no stone gives flat armour any more. The one place the flat number was better is a hit bigger than twice your armour, where the game subtracts rather than divides, and even there +2 only beat +5% below 40 armour.

On the heavy set idea: the game does not label armour heavy or light anywhere. The only thing separating them is the movement penalty each piece carries, so a set check means picking a threshold and then defending it, and a capstone that switches off because you swapped one helmet is worse than a smaller one that always holds.

So the fifth rank gives another 5% armour instead, on top of the 3% a rank. The stagger capstone goes with it, which is no loss: Steady footing gives stagger every rank and +5% armour at five, so the two stones are one each way round instead of both being half of each other.