Aha! I think I may get it.
When you convert a real to an integer, it rounds
down (so an armor value of 3.9 becomes 3).
A simple method of rounding up is rounding (real + 0.5) down.
My example of 3.9 becomes 4.4, rounded down, that's 4 (which is also 3.9 rounded up

).
2.4 --> 2.9 --> 2 (2.9 rounded down, 2.4 rounded up)
2.6 --> 3.1 --> 3 (3.1 rounded down, 2.6 rounded up)
Okay, so now I've made the trigger myself and this does work:
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Defense Break
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Events
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Unit - A unit Starts the effect of an ability
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Conditions
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(Ability being cast) Equal to Shield Break
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(Level of Shield Break Dummy for (Target unit of ability being cast)) Equal to 0
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Actions
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Set defenseTempReal = (Life of (Target unit of ability being cast))
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Unit - Cause (Triggering unit) to damage (Target unit of ability being cast), dealing 10.00 damage of attack type Chaos and damage type Normal
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Set defenseDamage = (defenseTempReal - (Life of (Target unit of ability being cast)))
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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defenseDamage Greater than 10.00
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Then - Actions
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-------- Armor is a negative value, setting it to 0 would actually benefit the target... --------
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Unit - Set life of (Target unit of ability being cast) to defenseTempReal
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Else - Actions
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Set defenseArmor = ((10.00 - defenseDamage) / (defenseDamage x 0.06))
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Unit - Set life of (Target unit of ability being cast) to defenseTempReal
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If (All Conditions are True) then do (Then Actions) else do (Else Actions)
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If - Conditions
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defenseArmor Greater than 0.50
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Then - Actions
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Unit - Add Shield Break Dummy to (Target unit of ability being cast)
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Unit - Set level of Shield Break Dummy for (Target unit of ability being cast) to (Integer((defenseArmor + 0.50)))
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Else - Actions
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-------- Armor is 0, reducing it by 100% is futile (besides, the first level of the spell decreases armor by 1) --------
Up to you to see where you went wrong now

(I hope you already changed the 0.6 to 0.06).
Test-map attached.