I positioned the inboard rear channel and drilled indersized pilot holes into all the ribs before drilling the rear channel doubler. The position of the rear ribs is pretty straightforward if you measure the distances on the rear channel and make sure the ribs are perpendicular to the spar as you go. Be very careful where you drill holes at this time. Some holes go through the riblets and other things that attach to the rear channel and rear spar attachment points. The final picture was taken later, but it shows how the doubler attaches to the rear channel and ribs 1-4. Also, as I was working on the rear channel, I went ahead and placed all rear riblets and drilled holes for other items while I was at it. I'll attach them later, but I had everything lines up and fresh in my mind, so I went ahead and did it.