Hi Gabrion,
As you wanted:
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ENHANCED SHADOW REALITY [General]
your shadow-based spells are more real.
Prerequsites: Spell Focus(illusion), illusionist level 1st.
Benefit: Your illusion (shadow) spells deal one-fifth (20%) more real damage if disbelieved. For example, a creature summoned with a shadow conjuration spell deals two-fifths(40%) of the damage a real creature of the same kind would deal if the target of the spell disbelieves it, instead of only one-fifth(20%).
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I agree that no matter how things stack up, reality should max out at 100%. So if the Powerful Shadow Magic ability adds 20%, and Enhanced Shadow Reality adds 20%, and the spell itself has 80% reality, it would do at most 100% of damage if disbelieved. I think it is fine, and can live entirely with that ruling. Really, it simply makes his lower level spells more effective.
So how will this work out?
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To give you a general idea of how his illusion (shadow) spells will often work out (with all of his class and feat abilities considered) is that he can convert any spell to to Silent Image (Full Round Action), which becomes (if he likes) an illusion(shadow) spell that can duplicate any evocation or conjuration (summoning or creation) spell of one level less. The reality of the spell will be 10% times the level of the spell slot used PLUS 40% (20% from Powerful Shadow Magic and Enhanced Shadow Reality each). Thus he can use a sixth level spell slot to mimic Cloud Kill (a fifth level spell). It will have 100% reality (6 x 10% PLUS 40%).
If he is standing on stone or earth, he can treat the spell as if it was a seventh level spell, and as if his caster level was six levels higher. The reality of the spell would be 110%, which of course is silly, so it maxes out at 100%, still.
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I hope this helps!