We nearly always use pinch sticks to record and transfer the distance between two points–in this case the span between the jambs of a door frame. It occurred to us that the addition of a slight tweak to the design of the sticks would also allow us to determine and record the midpoint of that distance. All you need do is bisect the width of the pinch sticks along their length. (Fix it permanently with a scribe line darkened with a sharp pencil of pen.) It is critical that the sticks be sized exactly to the same length and width and that the edges of the sticks are made exactly parallel. Some quick work with a handplane quickly makes it so.
To record the span, set the ends of the sticks to the jambs and then mark the location of the overlap on the bottom stick. Be careful to overlay the sticks so their edges are parallel to one another. When you transfer the span to a piece of wood (such as the weather stop molding in this case) you simply reorient the sticks to the overlap mark and make cutlines on the workpiece at each end of the pinch sticks.
The overlap mark also serves as an index for doing the construction to find the midpoint.
To find the midpoint, draw a diagonal line from the top of the index line to the bottom corner of the end of the stick. Note: you can skip making the longitudinal bisect line if you don’t mind taking the extra step of drawing a second diagonal from the bottom of the index line to the top corner of the end of the stick.
The intersection of the diagonal with the bisection line is the midpoint of the original span…simple as that.
Here’s what’s happening geometrically: We are finding the focal point of the circle that encompasses the rectangle created by the index line and the end of the underlying stick.
To mark the midpoint on the sill, lay the stick in place and mark at the line that you squared across the midpoint.