"While all common methods for resetting legendaries apply to shadow pokemon, certain instances (I dub them "shadow boosts" for lack of an official term) complicate matters. Whether it be one-under speed testing, Night Shade usage or the usage of the move Endeavor, certain attributes of the game change the required values for shadow pokemon away from what they would be if the pokemon in question was not of a "shadow" origin."
I'm not finishing looking into all of the "shadow boost" mechanisms, both for HP and speed, though if you want I can tell you what I've figured out so far. I don't have a working formula yet (I'd need the specifics on how to calculate IVs for that, which I don't have at this point), but the post in my trade thread (and sig) lists some of these values. At the moment I'm working on all of this, but there's still a lot of stuff that's inconclusive.