So there's a new 11-40 9-speed cassette from Sunrace, the CSM990 (SunRace | CSM990).

I am very keen to go to 1x9 on my Specialized AWOL at some stage in the future--probably when my FSA Omega Triple gets a little more worn, and/or the MegExo BB gives up the ghost.

I am wondering if anyone knows how I can calculate the required total capacity of a derailleur for a 32t chainring with an 11-40 cassette? I already have a 9-speed Deore M591 rear derailleur, but with Deore going to 10 speed, I might have to use an Alivio T4000 by the time I get around to converting. I have flat bars, so there is no problem with road shifters etc.

Would either of those derailleurs work (or an XT M772 for that matter)? They are all 9-speed and have a total capacity of 45T.