eng consoles dont stack 100%, there is a diminishing ratio, dunno how much
This only applies to the armor consoles. The way it work, effective HP is approximately linear.
Say you have 10,000 hp, and have two 10% armor consoles. If you equip one, you can take the equivalent of 11,000 damage. If you equip two, you can take the equivalent of 12,000 damage - which is only a 16% reduction, not 20%. You'd get the same result with one 20% console.
It's a bit wonkier than that if you get down to the finer details, and if there is a cap where no amount of stacking helps anymore. There's a spreadsheet floating around if you want to get into the finer details, but linear effective HP is a good way to get a rough idea.
Gives a good treatment of the DR formulae, and appears to sync with the tooltips in game. Of particular note is that captain's skills in Hull Plating and Armor Reinforcements effectively act like 'invisible eng consoles' in so far as they are affected by the DR formula as well. That means you're double-dipping against TWO diminishing returns formulae, so investing heavily in them is probably not the best plan.