Question about shield bash and gargoyle

So this is probably already common knowledge that champions which have shields that scale with max health can just activate gargoyle and then use their shield right before the duration ends so they get the bigger shield to protect them after gargoyle wears out and they get a stronger shield bash proc with no damage reduction to worry about.

My only question is, is the (bonus health converted to damage) part of shieldbash calculated at the time of when you get the shield bash buff (when you activate shield) or when you go to auto the enemy champs.

Because somewhere in between the time frame where you get the shield and you go to auto an enemy champion, you should be aiming to lose your gargoyle hp increase and outgoing damage % decrease, if the calculation was done when you activate the shield , you would also be gaining even more damage from activating it while gargoyle is active aswell.

Edit:Im confused about this because, in most cases, general damage calculations are done upon when you deal damage (like gargoyle and exhaust), with some being done once you receive their corresponding buff of somesort.