% or HP deficit in numbers This depends on healing style. Usually if you know your class well and exact healing output a spell has (in numbers) HP deficit in numbers can help. You see what amount you need and can act fast with the spell that can fill that up. However considering the hectic in nowadays raids you wont realy have time to do basic mats so %hp should be enough.
Incoming heals are usefull so you dont do that much of overheal/manawaste