200 seconds of attacks would mean you could use Shield Block 5 times total. Like you said that sometimes it’s not always used as soon as it’s up but you should be using it often so we’ll assume you hit it 4 of those 5 chances.

((10*4)/2)*104= 2,080

Since 40 of those seconds are guaranteed block we can’t count them in the rest of the math so we’re left with 160 seconds or 80 attacks.

80*0.20= 16
16*0.30= 4.8 (we’ll say 5)

11*52= 572
5*104= 520

So now in 200 attacks that brings the total damage reduction to 3,172.

20 Defense Rating will allow you do avoid almost half an attack (@0.48%) in those same 100 attacks and a 0.16% chance to prevent 1,000 damage. So assuming the boss hits for an average of 7k, 0.48% of that damage is 3,360.

So overall your gain from 20 Defense is relatively the same as the 40 block value in terms of the amount of damage it prevents. Of course really hard hitting bosses like Patchwerk change this entirely but the average boss doesn’t hit all that hard. The added benefit of the block value is that you not only “avoid” a comparable amount of damage but the amount of damage you take in is much more consistent and that’s a lot easier to heal than spiky damage (speaking as a former healer). And of course that added threat on Shield Slam, Damage Shield, etc are nice things as well.

Just my 2 cents and I’m bored as hell at work =P.