Find the Center of a Square or Rectangle in Perspective

Now line up your ruler so it meets the center of the square where the diagonals cross, and draw an orthogonal or 'vanishing line' through it to your vanishing point and extend it to the front of the box. Now you have the center of the front and back sides of your rectangle, dividing it neatly in half.

If you draw a vertical straight through the center, you'll have the box divided into half vertically as well.