Don't have the damage reduction be additive, have it cumulative.


 Damage = Damage *
 (1.0 - helmetReduction) *
 (1.0 - gloveReduction) *
 (1.0 - trousersReduction) *
 (1.0 - upperArmorReduction)

Each layer of armor reduces not the complete damage, but the damage remaining from the previous layer.