Naval bombardment vs units in a structure

[Pocus]

Is there a common rule here on how units in structures naval bombarded can avoid damages, with the structure targeted instead? Is it moddable, on a per structure basis?
I have noticed that ships in harbors avoid damage 80-90% of the time. This % is dependent on what? If the harbor is at 0 strength, does this protection apply?
Similarly, forts sitting on even a minor supply source are much (much) more resistant to damage. The issue is that sometimes they are on such resources just for the supply level (Pulaski for example to get supply 3, otherwise it would be at 0 given it is in a one-hex island). But the intent was perhaps not to make the fort more resistant.
So again, can this be modded, on a per structure basis? Or a tech increasing chance of dealing damages to structures?

[BillRunacre]

The % chance cannot be directly made contingent on the resource type, as it is controlled by one setting which can be found here:

Campaign -> Edit Country Data -> Edit Research -> Advanced

And there is a Resource Bombardment section on the right of that panel.

You'll note that there is a % setting for Defender Unit Hit Increments. This is the increasing chance with specific research categories of the unit in the resource being hit by the attack, IIRC from having Naval Weapons research.

[Pocus]

So here this is the rightmost & topmost value? And the one just below it is the increased chance per tech?
I'm still not clear which tech it is though, is it hard coded to naval research?

And what are these mysterious accuracies boxes?

[Pocus]

One extra important question. Once the resource is at 0, does it still give this protection ability to the harbored ship?

[Beriand]

And what are these mysterious accuracies boxes?

They are, well, accuracy loss beyond base range (per hex) If artillery unit has base range of 2, but through upgrades it increases its range to 4, then with these values as presented it shoots at 2 hexes alright, but at 3 hexes it has -10% attack strength and at 4 hexes it has -20% attack strength.

[BillRunacre]

One extra important question. Once the resource is at 0, does it still give this protection ability to the harbored ship?

IIRC, when it is at zero the ships occupying it will be hit 100% of the time.