By sub-rudders, are you referring to the added little vertical winglets added to the top and bottom of the elevator stab? And a flat bottom trainer wing is on a parallel line with the deck of the floats right? My book only explains the cub wing which isn't truely flat. Teaching my 10 year old to be a solo pilot seems easier at the moment with all the choices to make and right order to do things. Still have a LED light set to install before it gets covered also. So can I just assemble the plane with the step @ 1/2 behind the CG, wings parallel, then braces for the mounts, lights, covering, equipment, then balance at the new CG(book says it will move forward 3/8)? Is it that simple? Or do i have t maintain balance through out the whole build? Appreciate the help guys