Tony....
Here's another 'trick' that I learned the hard way when I first started using sketcher and had the same problem with open profiles:
Look for a toolbar called SKETCH TOOLS - it has 4 icons with the left one being a blue grid with an orange dot. Activate the two icons on the right (make them orange) so you automatically get Geometric Constraints and Dimensional Constraints. Doing this, and ALWAYS doing a sketch analyis really helped me.
Concerning the other two icons in SKETCH TOOLS: SNAP TO POINT is personal preferenced (I don't use it), and CONSTRUCTION ELEMENT can be used when you know you want to create several construction lines or points.