There are only a few dimensions you need to know to draw the garden path. The half-width is easy: it is just half the width of the path. You already defined the code to obtain this width from users and saved it in an association list.
Tile spacing is also easy; it is twice the radius (that is, the diameter) plus the space between the tiles. The dimensions are also obtained from users.
Row spacing is a little trickier, unless you are really sharp with trigonometry. Here is the formula:
Row Spacing = (Tile Diameter + Space between Tiles) * (the sine
of 60 degrees)