As far as i'm aware Determination does not equal you (healer) healing better, but affects how well your heals hit the other player, IE tanks with more determination will be healed better. For example, take 2 almost identical Paladins, the only difference is one has 200 determination and the other only has 100. My heal will affect the one with 200 better and heal for more then the one with only 100 determination. It of course adds much more then just healing, but that is how it affects heals