good way to make predefined placement points

I want to be able to have placement points on a cube i called foundation and i want the other cubes i try to place on it snap to those positions i already have the placement of them normal and all but what i need is a good way to detect witch part of the cube i am looking at and say example being if I'm looking at the one corner the preview i have snaps to that exact corner till i look at another part of the foundation i cant figure a good way to do the math or something any ideas please help.