found this in gameplay.constants
HullPointPercentageIncreasePerArmorPoint 0.05
what this could mean is that:
a ship with 0 armour and 1000 hull has 1000 ehp (effective hull points)
a ship with 10 armour and 1000 hull has 1500 ehp.
a ship with -6 armour and 1000 hull has 700 ehp.