I go by the following formulas...
Damage Reduction (what you see) = ArmourValue/(ArmourValue + 7.5*level).
Armour Value = DamageReduction*7.5*level/(1-DamageReduction).
After you account for + 20% def (via 6dot processor), a 25% def set bonus would yield 20.8% more armour. Similar to Offense the % impact of defense mods depends on the armour you get from mods.
For the first armour set you apply you could get net 20% more armour max.
The second set... (armor is at least 145% of base) you give 17.2% more armour max
The third set.. you give max 14.7% more armour.
So you get a bigger bang for your buck with one set over three.. And you get less bang for your buck if you have flat defense on your mods.
Assuming net 20% additional armour by using a defense set...
Current Reduction: 25% Value with Defense Set 28.57% Damage Reduction 4.8%
Current Reduction: 30% Value with Defense Set 33.96% Damage Reduction 5.7%
Current Reduction: 35% Value with Defense Set 39.25% Damage Reduction 6.5%
Current Reduction: 40% Value with Defense Set 44.44% Damage Reduction 7.4%
Current Reduction: 45% Value with Defense Set 49.54% Damage Reduction 8.3%
Current Reduction: 50% Value with Defense Set 54.55% Damage Reduction 9.1%
Current Reduction: 55% Value with Defense Set 59.46% Damage Reduction 9.9%
Current Reduction: 60% Value with Defense Set 64.29% Damage Reduction 10.7%
Current Reduction: 65% Value with Defense Set 69.03% Damage Reduction 11.5%
Current Reduction: 70% Value with Defense Set 73.68% Damage Reduction 12.3%
Assuming net 16% additional armour by using a defense set...(Basically if your thinking of adding a second defense set).
Current Reduction: 25% Value with Defense Set 27.88% Damage Reduction 3.8%
Current Reduction: 30% Value with Defense Set 33.21% Damage Reduction 4.6%
Current Reduction: 35% Value with Defense Set 38.45% Damage Reduction 5.3%
Current Reduction: 40% Value with Defense Set 43.61% Damage Reduction 6.0%
Current Reduction: 45% Value with Defense Set 48.69% Damage Reduction 6.7%
Current Reduction: 50% Value with Defense Set 53.70% Damage Reduction 7.4%
Current Reduction: 55% Value with Defense Set 58.64% Damage Reduction 8.1%
Current Reduction: 60% Value with Defense Set 63.50% Damage Reduction 8.8%
Current Reduction: 65% Value with Defense Set 68.30% Damage Reduction 9.4%
Current Reduction: 70% Value with Defense Set 73.02% Damage Reduction 10.1%
Assuming Net 14% additional armour (i.e. third set). Note armour value is with 2 sets applied.
Current Reduction: 25% Value with Defense Set 27.54% Damage Reduction 3.4%
Current Reduction: 30% Value with Defense Set 32.82% Damage Reduction 4.0%
Current Reduction: 35% Value with Defense Set 38.04% Damage Reduction 4.7%
Current Reduction: 40% Value with Defense Set 43.18% Damage Reduction 5.3%
Current Reduction: 45% Value with Defense Set 48.26% Damage Reduction 5.9%
Current Reduction: 50% Value with Defense Set 53.27% Damage Reduction 6.5%
Current Reduction: 55% Value with Defense Set 58.22% Damage Reduction 7.1%
Current Reduction: 60% Value with Defense Set 63.10% Damage Reduction 7.7%
Current Reduction: 65% Value with Defense Set 67.92% Damage Reduction 8.3%
Current Reduction: 70% Value with Defense Set 72.68% Damage Reduction 8.9%
Big question: Is taking 7.5% less damage per hit better then having 15% more health? Yes if you assume you lose more protection then health.