here are the assumptions (feel free to tweak the numbers)
Buff Dam Reduction Dam Taken
Barrier 45% 65%
Aura 20% 80%
Shield 20% 80%
voker shield 1 10% 90%
voker shield 2 10% 90%
voker shield 3 10% 90%
voker shield 4 10% 90%
voker shield 5 10% 90%
Stoneform 30% 70%
Stone Skin 25% 75%
so to determine how much damage you take you multiple each dam taken against each other. so someone with both barrer and aura would take 52% (.65 * .8)
so for the general example of volkov you have pre-stone skin taking 17.19% damage (thats an effective 82.81% dam redux) (the math is .65*.8*.8*.9*.9*.9*.9*.9*.7)
adding stoneskin to that you end up taking 12.89% damage (effective total dam redux of 87.1%)
Now I had originally thrown 5% as a number I came up with on the hip and it is somewhat vindicating to see that I was pretty darn close to where the actual math panned out. If you disagree about the relative damage reductions of the various affects then put in your own numbers and do the math but the fact remains that throwing 25% reduction on top of an already big pile of redux doesn't achieve as much as when you throw it on a small pile of redux. The difference is so small as to be irrevalent in the vast majority of cases (if you die before the invoker reaches 5% hp then it didn't matter)
