The point of impact would generally be the deepest part of the dent, along with being the first in, first meaning that it was first impacted. This can change by the type of dent you are trying to fix, say something scraped along the side of a vehicle leaving a long dent that got deeper as the woman continued to drag the car down the side of something.
Each dent is different so you have to look at direct, and indirect damage, direct is point of impact, indirect is the result of the blow, ripples stretching, anything along those lines. Once you get an idea of HOW the dent happened you just reverse the direction of impact to repair the area.
What you are confusing is that the deepest part of the dent is usually the first in as it is exactly where it was struck by an object.
