When the wing front to rear center line is horizontal as the airplane is flying, the top and bottom main spars are vertically over each other. Therefore the webbing when centered on the bottom and top spars is vertical.

On the plans take note of the cross sections of the wing at W1 & W2 and notice that the webbing is centered on each of the 3/16 X 3/8" spruce spars

also note on the plans about the middle of the right wing there is a shadowed cut away that shows the shear webbing centered on the spar

Picture #4 in the wing construction section of the booklet depicts the proper installation procedure. Notice to the right of the picture that the installed web is setting on the center of the lower spar and is centered with the notch for the top spar.