Originally Posted by
cxwrench
The 812 is meant for 1x, it has a capacity of 31t of chain wrap. Your setup requires 40t of chain wrap, and the 810 has a capacity of 41t. You stand a really good chance of tearing the derailleur off the bike if you go big/big with the 812...or you're gonna have a super saggy chain in small/small. Wait for the proper derailleur (810).
How much "safety" margin is built into these things? I am asking for curiosity's sake, because I personally don't run these things beyond their spec, but I see instances where people have done it, and seemingly they make it work by adjusting the setup, etc. Also, I do agree that the proper derailleur is the absolute safest way to go.