Question about +% fire/cold/lightning skill damage

[AwesomeRossum]

So this may be a dumb question, but I want to know where this is added.

Example (totally made up numbers, just to make things easier)

skill level 10 lightning, 1-100 damage

after adding skill level 10 chain lightning, 80% synergy, lightning damage 1-180

after adding skill level 10 lightning mastery, 100% bonus, lightning damage 1-360

Now my question, if I wear an eschuta's with +20% lightning damage, where does it get added? Does it multiply to the final total (after synergies and mastery)? Does it add with the mastery bonus and then apply to the skill? I guess I just don't know the formula

[DECOYBOY]

the 20% lightning dmg is just added onto the lightning mastery. therefore, not much of a boost. only cold dmg is boosted significantly by extra % cold dmg