Step 5: Determine the position of the first seat board
Partially hammer a nail in the very center of both the lower and upper frames.
This is the axis from where we can measure out the distances along the spokes for the tabletop seat board and the seat board placement.
When any tabletop or seat board is in place, the length of that board should be the same as the distance from the outside ends of the board to the center of the frame (the axis nail).
In effect, each board forms one side of an equilateral triangle.