Except for the end points, it is impossible to predict what a Laffer curve will look like. It can only be determined by trial and error and it is constantly changing.
It's sorta like trying to catch a butterfly but only being able to open your eyes for one second every minute.
Besides the end points, we also know (without trial or error) that virtually all points between the ends are more than zero, and that they're not all equal, which means that there has to be a maximum point in there somewhere. The way we know whether we're on the right side of the maximum (excessive rates and diminishing returns) is by cutting taxes (we did) and seeing if revenue goes up (it did).
Tell me if I'm missing something, but it sure looks to me that this proves that before the cuts we were over taxed.