As far as I know that is how they work. I am sure it isn't 4x - they are not THAT powerful in game.
SOSE uses a weird system for percentage effects - they actually do use the final effect as the initial value. A 75% damage reduction actually means 1.75x effective HP - this is why you can get buff/debuff values over 100%. A 100% damage reduction means half damage, 200% 1/3rd, 300% 1/4th, etc.
For buffs, the formula is:
Xnew = Xold (1 + Buff)
For debuffs, the formula is:
Xnew = Xold/(1 + Buff)
In this way, buffs and debuffs of the same percentage value cancel properly. The Nebulon-B takes a little more than half damage compared to your average Imperial space wedge.
Erm, that's not how percent damage reduction works. Your calculations are what it would look like if the ability gave a 75% increase to shield and armor health.
At a 75% reduction every 4 points of damage taken is deducted as 1 damage. Making the 5,150 combined shield and armor health need four times as much raw damage to grind down.
Hence 400% increase. 1/(1-% damage reduction) * (shield+health)
I'm not sure where you're coming from with the 400% increase. Since the ability offers a 75% damage reduction, 75% of incoming damage should be taken away. That would mean the effective hit points of the ship's hull and shields should increase by 75%, meaning from 5,150 to 9012.5.
No I did not, thank you. Despite my efforts, math has not been nor ever will be my strong suite. So that would mean they have a whopping 400% increase in their effective hitpoints.
The Assault frigate at 20,600 can handle more of a thrashing than most capital ships. Only ones I would expect it to not beat out(due to other damage reductions like armor and armor type) in tankiness are Praetors, MC90s and Lucrehulks. That is absurd!
Sure. When you can replace it with Bothan Assault Cruisers it doesn't seem all that imposing. In actuality though the Nebulon packs quite significant punch with its turbolasers... to the point where unless the BAC is using its gun batteries on both sides it almost gets outdamaged by 2 Nebulons. In terms of turbolaser damage only it actually loses in this regard. They're also tankier than the Bothans which is kind of silly.
How does it work exactly? If it works how I might think it would work and gives every attack made against it a 75% chance to not deal damage, that would mean the have 300% increased EHP. There is a bit of RNG involved sure so it could be a little less or a little more. Still 6600 EHP on such a cost effective and low population unit like the Nebulon and however much it is on a workhorse like the Assault Frigate seems quite over the top.
Edit: I looked in the files a bit more and my assumption made above seems to be true. The Assault Frigate has 2090 HP and 3060 shields. Giving it a grand total of 15,450 EHP. No wonder it takes my fleets 3-4 minutes to kill NR starting ones when I encounter them early in the game.
It treats the value as a simple damage reduction.