Question nearly on this topic: Is Front Defense is different from Melee/Ranged/Magic Defense with regards to the damage reduction cap?
From the damage formula, Front Defense is additive with Melee/Ranged/Magic Defense (for non-flanking attacks) as a separate stat, but what I have not been able to tell is whether Front Defense and Melee/Ranged/Magic Defense are counted cumulatively toward a general 80% Defense cap or are considered separate. I'd guess that they are cumulative, but I have not been able to get hard data on it since my promotion are so low...
Formula
final_damage = (base_damage * rand(0.95 to 1.05) - armor * armor_multiplier * (1 - armor_penetration))
* (ability_multiplier + attack_bonus + flanking_bonus + level_bonus + difficulty_bonus + rank_bonus)
* (1 - magic_resistance)
* (1 - min(defense + front_defense, 0.8))
clearly states that front_defense adds to defense and 80% cap applies to the sum of both.
If you want to test it...
either find someone with high enough promotions or do it on single player where you can from my experience craft gear which can reach any defense cap you want.
Probably easiest way to test it will be with turn the bolt.
Shield gives 30% front, this 50% front. So in theory you are capped. Turn the blade which works also on range should not give anything here vs ranged.
Eventually you need around 50% range or melee defense, shield can give 10% even of one this stat + armor upgrade + 12% + (if melee) 6% + around 15 con from passive another 7%.
Probably melee would be easier, with proper gear you should have around 35% melee defense, so only 30 promotions are needed.
With 50% as base melee defense and shield, from front damage should be lower, capped and taking passive like turn the blade shouldn't reduce it even more.