First off, get your point of view normal to the view with the lines you are trying to dimension vertical or horizontal.
Second, try hitting alt and or shift while placing the dimension.
I have a shape I am trying to dimension. Two faces are parallel but will not dimension relative to each other, it always auto selects the end. I've tried various options but none are working. Any suggestions? Thank you.
What you need to do here, is "Lock the Dimension Plane" and set it to the front face.....there is a button at the right side of the ribbon for this.
Design Manager Streetscape Limited
Solid Edge ST10 [MP2] Classic [x2 seats]
Another way to create this kind of dimension with the distance between as you did in your first attempt is to define a dimension axis. Click on the button "dimension axis" and select a line then proceed to create your dimension.