You may have to determine your average chord for you biplane. This is simple. Level out your plane, and stand a large square up against the L.E. of your top wing. Then measure from the square, to the T.E. of the bottom wing. This will give you the average chord of your bipe. Think of it as looking straight don onto your airplane. The width of the wings that you see, is the chord width you use to calculate the chord. Hope this made sense.