We've had a question like this before. I don't recall the title exactly. The calculation is only for the exact spot where you are located right now (or at some hypothetical point in the flight.) It has nothing to do with drawing long lines all the way across the map. If your track crosses multiple lines of declination then you'll be repeating the calculation at multiple different points during the course of the flight or during the course of the flight planning. But you were asking for one specific point so there is no need to draw a line or think about more than one point. (A line is a collection of an infinite number of points.)
Why are you drawing a track all the way across the map from 70 N to 70 S? And why did you draw it along a track line of around 150 true (or 330 true if you are going the other way?)
That would be an interesting flight to make but it is not related to the question you seem to be asking.