Alright, I know when you interact with a shadow spell, you get a save to reduce it's effect. Let's say you cast mage armor or ablative barrier...
Invisible layers of solid force surround and protect the target, granting that target a +2 armor bonus to AC. Additionally, the first 5 points of lethal damage the target takes from each attack are converted into nonlethal damage. Against attacks that already deal nonlethal damage, the target gains DR 5/-. Once this spell has converted 5 points of damage to nonlethal damage per caster level (maximum 50 points), the spell is discharged.
The person you cast it on chooses to fail their save, so it's 'real' for them. When someone strikes you, do they also get a save to reduce the effectiveness of the spell to 20% of normal? (thereby reducing the armor bonus to +1 and the damage blocked per attack to 1, instead of five) Or would the only person that gets a save be the recipient of the spell?
Well, if they strike them with a weapon, the weapon automatically makes it's save, so there's the 20%. if they use a natural weapon, the save comes into play. I like how this guide talks about doing Shadow spells, instead of just rolling 20% to see if it's fully effective, by reducing the effect to 20%.
another example is fire shield from shadow evocation. someone hitting you would get a save to reduce the retributive damage to 20%, but when they shoot you with a fireball, you still get the reduced damage for the protections of the spell, so no save on their part there, as you chose to fail your own save, and get the full effect of the spell.
Hmm so the rule of thumb is when some one directly interacts with it, they get a save to reduce it to a certain percent of it's effectiveness. Sounds good.