if an item says it gives 10% patk / P Atk Crit DMG / P Skill Crit DMG / P Skill Power then that means you will do 10% more dmg on target respective to the stat when it is applied.
you have items that say: patk/pskill crit dmg <- that means normal and pskill critical %, then you can't apply both damages on the equation.
(there is also things say: normal/pskill crit dmg, is the same as above they just didn't standardize all the items)
i'm not following you and i would want to understand why mutiplicative and not additive.