I didn't unlock Selene yet, just theorycrafting. My question is for the heavy duty armour mod, which provides a percentage based hull boost. Is this percentage additive or multiplicative with the boost provided by the type of armour? Or in other words, does the mod take my base hull value and adds its boost absolute value to my existing value, or does he take my final (existing) hull value and applies its percentage to that?
Most of my ships I've put the Heavy Duty mod on the lightweight armour, since it adds zero mass. But my FAS and Gunship, I've modified the Military armour. I'm fairly sure that the increase is much more than it would have been for the lightweight. So I'm fairly sure it's on the final hull value. Adds a lot of weight though.
You may find this armour calculator useful. Made it a little while ago now, but it kinda slipped through the cracks: https://forums.frontier.co.uk/showthread.php/301311-Armour-HRP-Mod-Calculator
You CAN go to that armour thread and bump it with your results.
I find it a bit telling on the subject of game balance that the shield calculator thread has almost 90 replies, and the armour calculator thread has 0...