So I believe the below formula yields almost exact results:
Damage Reduction % = Modded Defense / (Modded Defense + (level * 7.5))
where Modded Defense = Floor(Floor(gear_level_armor_rating_compoundable + agi_armor_rating + str_armor_rating) * Def % multiplier from mods + Flat Defense from Mods + gear_level_armor_rating_noncompoundable)
gear_level_armor_rating_noncompoundable is any direct defense added from the current level of gear.
gear_level_armor_rating_compoundable is any accumulated defense stats added from previous gear levels other than the current
agi_armor_rating = Agi * 0.07
str_armor_rating = Str * 0.14
e.g. Fives has 15 armor added on gear level 10; this is non-compoundable. However, the armor from +30 str +5 agi added from equipped gear on g10 is compoundable.
Source: https://www.reddit.com/r/SWGalaxyOfHeroes/comments/52n4mc/brute_forced_defensearmor_calculator/