I don't think it actually heals a fixed number. The tooltip would indicate that it restores the player to full, regardless of health deficit. Considering that it also ignores status debuffs that lower the amount of healing done, I think it's less of a number and more of a percentage (though it shows up as a literal number in the floating combat text).
Benediction is a "Get Out of Jail Free" card that should be used for just that - someone's gonna die if they don't get healed up right now. With that in mind, I use it when it needs to be used and I don't worry about the Hate/Threat I might pull. Because if the situation is that bad, my first duty as the healer is to make sure people don't die (even to their own stupidity -_-). Period.
