You may be thinking of the Laffer curve (https://en.wikipedia.org/wiki/Laffer_curve https://en.wikipedia.org/wiki/Laffer_curve). Though my understanding of it

You may be thinking of the Laffer curve (https://en.wikipedia.org/wiki/Laffer_curve https://en.wikipedia.org/wiki/Laffer_curve). Though my understanding of it is that it doesn't prescribe any singulary "point" like 40%, but just says that there is a point somewhere and that this is the relative behavior of total tax collection around it, wherever it exists.