I have reported this 100 times if not more since beta.
The ACTUAL bonus in healing comes closer to someone being at 20% hp really after testing it a bit.
DOH...thought something was wrong...IDK, its just another spammable, next to Dwaynas Kiss and Orison of Healing. Guess I'll just go with spellbreaker then.
The targets health has to be below 50% after the first part of the heal and your DF bonus is applied. If both of those are applied and their health is still below 50% then the second heal kicks in
Although it should be changed so that DF healing is added AFTER the full WOH process simply because it is annoying for pure healers such as myself, who have high Devine Favour.