Historical Rules · 1 October 2009
Rule 806.9c
Official source text from the 1 October 2009 Rules release
If an effect would set a single player's life total to a number, that player's individual life total becomes that number. The team's life total is adjusted by the amount of life that player gained or lost. Example: In a Two-Headed Giant game, a player on a team that has 25 life casts a spell that reads, "Your life total becomes 20." That player's life total is considered to be 13 for the purpose of the spell, so it becomes 20 and the team's life total becomes 32 (25 + (20 - 13)).