There can't be any set amount of +healing required for any instance (unless there is very specific encounter). Raid composition can be so different. How many healers you got? Which classes? How well geared your tanks are? How is your dps doing?

How skillful and experienced your raid members are to get best out of your characters and team work? Even if you know these parameters, giving out any exact requirement is hard. If you don't like pulling numbers out of your bum, that is.