Right now even 18% wouldn't really be used, Stoneskin just costs too damn much mp to be used in combat. Stoneskin's only real use is that it is an aggro free shield if cast prefight and can tank 1 hit while the healer gets in position to heal.

Notably on Alex 1 normal a 10% stone skin will take 1hit of the AoE so it saves mana and the Assize for the second one, presumably when you will get mana back as you should have spent a little on the tank.

Also had a FC mate who had almost all his gear break in the middle of a Neverreap run so he had only 4k hp, stoneskin after every hit kept him alive throughout the whole thing, it was... Interesting.