Using the Brutix as an example:
“Gallente Battlecruiser bonuses (per skill level):
10% bonus to Medium Hybrid Turret damage”
What is the standard equation for calculating such things?
If I am at level 3 and the damage is, for brevity, 100hp, is it:
If 10% of the base is 10, is the level 3 damage 130?
or does it snowball forward where the level 3 damage is 133.1?
And, regardless of which method it is, is this consistent among all skill calculations?
Typically, within a skill, bonuses are additive. In your example the result would be 130.
I would not rule out exceptions. I believe I can recall a trade skill at one point that did recursive multiplication, but it clearly indicated such in its description.
Also, typically, bonuses from different sources are multiplied, so if you had 3 different skills each providing a 10% bonus separately, your damage would be as per your second example.