Base difficult of a ribbed hauberk is somewhere around 700 ranks.

I'm assuming you want it masterfully-crafted? With techniques and proper tools and 20 workability on mammoth bone, I'd put the difficulty of making absolutely sure you get a masterful piece around 800-850 ranks.

Mammoth bone is 2 workability, not 20 I think. I'd probably not try this before 900 ranks with WOTM. Honestly you're better off both in difficulty and hindrance in making smaller pieces.