Make a Line 8 inches from the Edge

OK so in AutoCad you can easily make a dimension line that is a shape 8 inches from a ledge. You make a rough shape. then dimension it using the tool… How do i do this in sketch up

Can you show what you are trying to do? Maybe a screen cap from ACAD, or a hand drawn image? As it is, it is hard to envision what you are trying to accomplish.