Before entity attacks trigger, can you change the amount dependency?

So, I'm trying to make an armor that reduces damage from certain entities, so in order to do this I tried to use the "before entity attacks" trigger and noticed it had a dependency called "amount", of wich you can extract a numerical value from with the "numerical dependency" procedure block, I tried several ways to alter this dependency, but to no avail. so is it possible to change it? if it isn't, do you have any ideas on how to reduce damage taken, aside of adding the resistance potion effect wich is the only viable method I found. The reason why I don't like applying the potion effect is that when the entities hit you, you get resistance to everything, not just those entities.