it depends on the amount of resists u have. Lets say u have 95% resists to lighting and take 1k of actual damg ur only taking 50 damg after applied resists.
20% of 50 damg is only 10 points of damg healed while tgods is +20, 20 > 10 am i not correct? so what im saying is tgods is better than a wisp projector unless you dont have a tgods and only have a wisp