Fiora Passive true damage calculations

The wording on a lot of the descriptions of Fiora's passive indicates that there's a step up of percentage per 100 bonus AD, but I've been assured that it's scaling without a definitive answer. The verbage I've seen on her wiki is:

>Basic attacks and/or damaging abilities against a Vital deal 2% (+ 4.5% per 100 bonus AD) of target's maximum health as bonus true damage, heal Fiora for 30 - 115 (based on level) and grant her 15% / 20% / 25% / 30% bonus movement speed that decays over 1.75 seconds.

So, for example, if she has 219 bonus AD, does she do 2 + (4.5 x 2) percent max health true damage, or does she do 2 + (4.5 x 2.19) percent?

In the (incorrect?) former of that example, if she then buys a Bloodthirster and now has 299 bonus AD, she would still only be doing 2 + (4.5 x 2) percent. If it scales smoothly, would it be better to have the wording be:

>Basic attacks and/or damaging abilities against a Vital deal (2 + (4.5 x 1% Bonus AD)) percent of target's maximum health as bonus true damage, heal Fiora for 30 - 115 (based on level) and grant her 15% / 20% / 25% / 30% bonus movement speed that decays over 1.75 seconds.

Just looking to clarify for my own purposes. Thank you!