My Mega Latias set is competitive and will be different than the ones you see on this page.
Latias (F) @ Latiasite
EVs: 252 SpAtk / 4 SpD / 252 Spe
Nature: Timid (+Spe, -Atk)
The Dragon Pulse is for STAB in case you run into a type that resist Psychic. Your 252 EV speed will let you go first (unless they are scarfed) with Calm Mind and your 252 EV SpAtk will make you hit hard. I know Stored Power is and odd choice but the formula is 20 + (X * 20). So if you get two Calm Minds, X = 4, meaning that Stored Power will do 100, and with STAB 150. The one thing that recks Stored Power is a stat removal move such as Clear Smog and Haze. Protect is there to prevent that from happening and can also come in handy when dealing with scarfed mons that are a danger (Weavile, Tyranitar, etc). I hope this helps.