Fricket, I just did some quick math for you.
If you don't even look at block value or SS, taking IDA nets you 14 less damage than not having it at 19000 AC taking unmitigated hits of 3500.
Lets just say 15 less dmg for easier math.
Shield spec provides 30% more BV. In order for SS to match IDA, you need to have 15/.3 = 50 BV before SS.
Lets look at some other block values and what you get from SS:
100 => 30 extra BV
150 => 45 extra BV
200 => 60 extra BV
250 => 75 extra BV
300 => 90 extra BV
400 => 120 extra BV
500 => 150 extra BV
100 BV has double the effect of taking IDA
200 BV has quadruple the effect of taking IDA
300 BV has 6x the effect of taking IDA
You can see how quickly it scales.
Consequently, if you were at 20k AC instead of 19k, then IDA would provided less "extra" (13.125'ish less damage rather than 14).
Just some food for thought.
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