Could use some clarity on a few Item/Mastery tooltips

Alot of masterys have the simple description "deal x% more damage" with no explanation as to how, it would be really usefull to know how that increase is applied eg: is the buff before/after mitigation, does it only apply to base damage or does it scale with items, is it multiplicative with other masterys or additive

That's just a few examples to flesh the idea out, another problem i have is with Lord Dominiks Regard:

Unique – Giant Slayer: Grants up to +15% bonus physical damage against enemy champions with greater maximum health than you (+1.5% damage per 50 health difference, maxing at 500 health difference).

It strikes me as really unspecific, more eg: does the 15% apply only on auto attacks or all damage i output on them, does the 15% get calculated before/after mitigation, what if im a mage dealing AP dmg, does that 15% only scale off AD damage or will syndra's Q proc it too, and deal some mixed damage

I'm trying to make this not sound like a rant, this is just asking for some clarity. If any rioter knows the math on these things (or a really dedicated player), an explanation would be met with my sincere thanks & all the fish you could eat (no promises on the fish)