An attentive reader asked: “What if you wanted to lay out the centers of the slats instead of their edges”. A great question as it is actually often preferable to identify the centerlines for laying out the joinery–such as the mortises for the slat’s tenons in this case. With the centerline marked, all you need do is index a miniature story stick representing the length of the mortise to define its boundaries. Again, this would likely be done on a full length story stick representing the rail itself.
As you can see in the drawing, you locate the centerline of the first and last slat by setting the dividers to the gap width plus half the width of a slat. Next, reset the dividers (or use another pair) to the spacing of one slat plus one gap. This is the distance between the centerpoints of the slats. Of course, you could use algebra and I could come up with the formula for you to follow but I’ve moved on to the fun part of cutting mortise and tenons!