For a triangular pattern, all wells are a distance D apart, creating equilateral triangles.
Note that the wick drain grid drawing is just a representation of the spacing. The horizontal distance between wick drains is the spacing defined by the user, and the drains are drawn in rows a vertical distance apart also equal to the user-defined spacing. They are offset from each other in alternating rows only for aesthetics. How the wick drains are drawn on the screen has no relationship to how they are implemented numerically in the analysis engine.