I disagree that there is anything mathematically wrong about looking at an absolute percentage-point change in addition to looking at the corresponding proportional change. They measure different things, and thus are both valuable.
When considering a binary event, such as whether a d20+bonus roll hits a DC, the absolute percentage-point change provided by an Effect (e.g. advantage) is an accurate measure of how often the Effect will be outcome-determinitive. This is useful information and is not in any way "wrong".
The "wrong" part was converting a normal distribution to a mean-shifting a flat distribution. The is wrong.
Evaluating how often a the "Effect" (a nebulous term that I can easily define in a way that makes this wrong, but am choosing to assume that you're mostly referring to ad/disadvantage) is useful has little to do with the size of the absolute percentage change. To consider needing an 11, disadvantage would mean that the normal 200/400 chance drops to 100/400, or the Effect is determines the outcome 100/400 times more than the normal. If you need a 20, though, the normal 20/400 chance becomes 399/400, or the Effect determines the outcome
379/400 times more than the normal.
Furthermore, the absolute percentage-point change in success rate of a binary event often provides more information than the proportional change when either are presented alone. This is because the practical significance (in the colloquial sense) of the absolute change is dependent only on the practical significance of success. By contrast, the practical significance of the proportional change is dependent on both the practical significance of success and on the base rate of success.
It does not provide more information. You're assuming facts, such as what is needed to be rolled for the absolute change while ignoring them for the proportional change. On even footing, neither present more information than the other. Frex, If I have an absolute change of -10%, and need a 20, then I will never succeed. You must also know the target number for the absolute change to have any meaning.
Luckily, the point I was making is not that the absolute change lacked information, or that the proportional change conveys more, but rather that the full sum of the impact requires BOTH to be understood. You may recall that I said, in the part you snipped of my post above, that you have to go deeper to get the full understanding and that stopping at the absolute change will confuse you? Because, as illustrated above, you do. Proportional changes are not sufficient by themselves, either, as they require the additional information to ground them and provide the needed context.
As I conceded to [MENTION=6811643]Krachek[/MENTION] above, even this doesn't determine optimal usage in game, as the context of the situation is even more important than the math. However, the math does clearly show that the tails have the greatest impact absent that context. In game, it depends.