[DeathsSilkyMist]

Did one more test without any shield in back or secondary. All tests are 1000 hits. Reposting the first two for easy viewing. This is against https://wiki.project1999.com/Shiel_Glimmerspindle .

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363 AC Test, Nothing in Back Slot, Nothing in Secondary Slot
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DV, Count
19, 373
22, 37
25, 33
28, 31
32, 25
35, 45
38, 26
42, 37
45, 32
48, 36
52, 30
55, 32
58, 41
62, 27
65, 44
68, 29
72, 34
75, 46
78, 33
82, 9

Total Damage = 39093

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363 AC Test, Lodi Shield in Back Slot, Nothing in Secondary Slot
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DV, Count
19, 381
22, 22
25, 33
28, 28
32, 35
35, 39
38, 27
42, 37
45, 37
48, 38
52, 28
55, 32
58, 23
62, 41
65, 41
68, 41
72, 44
75, 31
78, 33
82, 9

Total Damage = 39236

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363 AC Test, Nothing in Back Slot, Lodi Shield in Secondary Slot
=========================================

DV, Count
19, 418
22, 29
25, 35
28, 45
32, 29
35, 22
38, 34
42, 36
45, 32
48, 33
52, 25
55, 29
58, 37
62, 32
65, 30
68, 31
72, 29
75, 30
78, 40
82, 4

Total Damage = 37110

This shows that shield AC in the secondary slot does affect mitigation. This is due to the shield AC increasing your softcap. Sadly shield AC in the back slot does nothing. The NPC I was fighting is level 40, so this means the "squelch point" is not affecting the result. My current thought is that the "squelch point" isn't an actual mechanic.