Question on (+% Healing and Shielding Power) with AP?

First of all thank you for reading my topic. I've been gone from LoL for 5 years and this mechanic is new to me.

I tried buying a (+% Healing and Shielding Power) item but they do not update the tool tip of the spell. So my question is, what is the interaction with such Heals and Shields and the (+% of Ability Power) coefficient. Is the (+% Healing and Shielding Power) calculated with the flat values of the heal or shield, or total value after (+% of Ability Power), or just increasing the (+% of Ability Power?)

Example, Shield Rank 1: [80 Shield] (+70% of Ability Power) with 100 AP. How does an Item that increases (+10%Healing and Shielding Power) effect the spell?

E.g. 1: [165 Shield (80 + 70% of Ability Power) + (+10% Healing and Shielding Power)] = Applied after the AP is calculated.

E.g. 2: [160 Shield (80 + 80% of Ability Power)] (+10% Healing and Shielding Power) = Applied to AP coefficient.

E.g. 3: [158 Shield (88 + 70% of Ability Power)] (+10% Healing and Shielding Power) = Applied before AP coefficient is calculated.

Or none of the above? Also to my original point, are multiple instances of (+% Healing and Shielding Power) added together additive or multiplicative? Also, does the (+% Healing and Shielding Power) increase the damage bonus applied by Shield?

Math is not my strong suit, so sorry if it's FUBAR. I'm just looking for the interactions to get an idea of how much the Passive is actually effecting my Shields.

Thanks for your time, and sorry if this has been asked before or if this is the wrong board for this.