THEREFORE: TO LOCATE A MIDPOINT QUICKLY AND ACCURATELY - USE DIAGONALS.
Equal Spacing By Diagonals 
To draw equally-spaced receding elements such as lampposts, first sketch two of them between the desired top and bottom guide lines leading to their vanishing point.
Now let's develop this further in side view (far right).
Step A: Draw diagonals between (1) and (2) to determine midpoint. A horizontal line through this point gives us midpoint of (1), (2) and all similar verticals.
Step B: Draw diagonals from (1) through midpoint of (2), to locate (3). Since the diagonals place (2) exactly midway between (1) and (3), the location of (3) must be correct.
Step C: Subsequent equidistant verticals are located by similar diagonals. (Note: It isn't necessary to draw both diagonals. One of them, used with the center line, gives the same result.)
The application of these steps in perspective will assure equally-spaced elements drawn with proper convergence and foreshortening.
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