There's been a question regarding the first sentence of my explanation of one way to make sure your wings are straight.

To elaborate, select which ribs will be the tip and root ribs. On those ribs, draw a straight line from the front to trailing edge. The line you've drawn represents the airfoil's chord line....we are simply using it as a reference line for building purposes. On symmetricle airfoils, the line you've drawn will be right down the center of the rib.

If you are building a flat bottomed wing, simply use the bottom of the wing as a reference and tack glue your 3/8X3/8 "shelf" flush with the bottom edge of the rib.

On semi-symmetricle wings; outline the root and tip ribs on butcher paper. Plot where the trailing edge and leading edge tips should be. Draw a line from each point. That will establish the airfoils chord line which you will be using as that rib's reference line .