I have fond a somewhat easier way of scaling... Yor simply divide your current scale with the desired scale and multiply that with the current wingspan. In this case 20/3 * 18,5 (current wingspan) = 123.33in, or 3/20 * 123.33 = 18,5in to scale form 1:4 to 1:20....Easy
In addition you can easily calculate the area of the wing after scaling. The current area * scale ratio²... If your plane currently have 30sq.in and you scale from 1:20 to 1:3.... 30sq.in * 44.44 = 1333sq.in