Virtually every tool in SketchUp can take advantage of object snaps. These are reference points that allow you to specifically select an exact point in the model without dealing with dimensions. I suggest learning how to use them and always use them when possible to increase your efficiency and accuracy when modeling.
For example, if you are using the line tool you can start that line on any endpoint, midpoint, intersection, intersecting plane, or various other reference points in the model. With a tool selected hover your mouse over the area you wish to reference off of and your options will be displayed on the model. The most common is a green circle that represents the endpoint of a line. A blue circle represents the midpoint of a line. And a red x represents the intersection of two objects.