s2's distance+ method works, but I have never learned to love it myself; I am an 'origin mover'. Use Option(alt)+Shift to locate the user origin on the point you want to refer to. Make sure the button in the Coordinate Box is 'up'. Now you can simply type X (input) or Y (input) or both, then Return/Enter, and you can precisely control the start or end of anything.
Either way works once you get the hang of it; matter of taste in the end.