Bill,

I think we all just eyeballed the measurement. I guess they don't give a top wing measure due to variations in building. If you really want to be on the money, just set the plane on a table with the wings a 0 degrees (block the tail up). Use a square or a piece of paper butted up to the LE of the bottom wing. Make a mark on the top wing and measure your 20mm from there.

Mark