A qd should get whatever residue is in the scratch.
Any polish with diminishing abrasives will be most "potent" at the start. After that it becomes more and more mild. Scratch X has dim.abrasives. In order to get the scratch leveled the quickest, you would apply a bit, rub a bit, apply some more, rub some more, apply..... I'd say you should spend less than 10 seconds per application. Also, don't rub hard. Repeated light applications will be less likely to introduce more marring. 10 applications isn't out of the question, depending on the severity of the scratch. As it looks pretty mild in the pic, it shouldn't be too bad. However, you said you can feel it with your fingernail, so it may be deeper than anticipated. It may be better to just make it "less there" than "gone." (save the rest of the paint.) After that, you could use a glaze and a wax and you may not even notice it from most angles.