Block Chance and Unlucky

Hello, i want to make some new project Build as Gladiator focusing on Block mechanic, my Glad will have 83% block chance i wan't to use new Chest [The Iron Fortress], this Chest have "Chance to Block Spell Damage is Unlucky" so if im use Gladiator [Versatile Combatant] i will have 83% spell block, but how unlucky from chest will change my spell block, how i can calculate my Spell block when it's "Unlucky"

Lucky/Unlucky means that the random number is generated twice, and the better/worse of the two rolls is accepted.

At 83% Unlucky spell block you are required to succeed twice in order to successfully block (otherwise if one fails, that one is used and you fail overall), so your real chance to block is .83 * .83 = .6889 = 69% effective block chance.