3AM max damage reduction stacking brainstorming
 Zealot Build - Warhammer 40k: Darktide 

NOT TESTED OUT JUST 3 AM AUTISM AND VERY FAST SLOPPY MATH

just a thought experiment of stacking damage reduction, i highly doubt it could be any useful at most if you can upkeep shroudfield's invigorating revelation
so either try martyr's purpose or second wind depending on how much it really reduces damage to take minimal damage and even then second wind might be better since the dmg might not be rounded down to 0 in the best case so just constant dodging and restore more toughness than they can dmg you
i have no clue how much it actually is reducing it since toughness and damage reduction are separate and should stack (assumed they just multiply if one is only taking toughness damage) 

DMG reduction: Good Balance (25%) * Shield of Contempt (60% but on average its just 24% over the game during constant damage) * Invigorating Revelation (20%)

Toughness reduction: Enduring faith (50%) * Benediction (15%) * I Shall Not Fall (idk if its 6,5%^7 or 45,5%) * keynotes (5% * 5% * 5% [sadly missing another 5%]) 

So lets say we take 100 Dmg to a max toughness shield (as in no health dmg):
 

Damage reduction first: 100*0,75*0,4*0,8 = 24 (on fast avg. 100*0,75*0,76*0,8 = 45,6)
then Toughness reduction: 24*0,5*0,85*0,545*0,95^3 = 4,77 [if 6,5%^7: 24*0,5*0,85*0,935^7*0,95^3 = 5,46] 
(fast avg.: 45,6*0,5*0,85*0,545*0,95^3 = 9,06) (fast avg 6,5%^7: 45,6*0,5*0,85*0,935^7*0,95^3 = 10,38)

10 *0,75*0,4*0,8*0,5*0,85*0,545*0,95^3 = 0,477 [10 dmg could be rounded down to 0]
9 *0,75*0,4*0,8*0,5*0,85*0,935^7*0,95^3 = 0,49 [if 6,5%^7]
here we cant use the avg since only in those 4 sec will we reach the max reduction
100 *0,75*0,6*0,8*0,5*0,85*0,545*0,95^3 = 11,92 [6,5%^7: 100 *0,75*0,8*0,5*0,85*0,935^7*0,95^3 = 13,66]
in those 6 remaining sec

[LUNATIC SLEEP-DEPRIVED RAMBLING BELOW]

Also would be interesting to see how one could incooperate Restoring faith (if it were restoring 20% of the DMG before reduction, it would be busted with that if it can stack)
like if it really would work in that op way then Enemies within, Enemies without would be just a good effect since it will counteract in those 4 sec though still other talents would be better if one had more points since dodging for 15% toughness is straight better
i also left out Bleed for the Emperor since idk how to incooperate with that idea does 0 wounds count as a threshold for max effect? would it be then better to stay in 6 wounds? and it cant regenerate the threshold if one does not want to reset I Shall not fall 
BUT if restoring faith heals also health and not just toughness then it could [IN MY BROKEN IDEA of restoring faith [I NEVER USED IT SO NO IDEA]] be cool since you will always regen more than you take after those 4 sec so we could proc Bleed for the Emperor often if we aint dying instantly :P