Have you tried to take that piece of paper and make it lay flat, on all sides, on your head?
Try it.
You'll see that as you get the two sides (left & right) to lay flat towards your temples, the middle of the front will buckle up.
Have you looked at front contour tapes?
There are several options in shapes that mimic natural front hairlines.
Not one of them is the shape of the straight line of your template.
Most people's hairline recedes. The idea of the partial is to bring it forward again.
Therefore the internal line of your horseshoe diagram represents where you still have hair and where it has receded to.
I would have thought that what you're looking to do is bring that same hairline (ie the internal line of the horseshoe) forward.
So the front of your template would normally be parallel to that.
I hope that makes sense?