It’s been posted elsewhere but to summarize: we calculate the team power of the top 30 heroes of a player and sum them together. We give extra weight to the top five heroes with the simplistic assumption that they will be used as the defense team. Then we sum those numbers together for every member of the alliance. That’s it.
If you tell the name of your alliance and the date of the absolute worst match you’ve had, I can check it out later in case it offers some new insight to us.