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This is just a test setup to share my thoughts. First I drew the center line and then set the currently uncompleted Center Spar flush with the back edge of the doubler. Using the existing holes, I marked the longeron location. I don't want to pre-drill these holes, this might be better from the bottom.
I marked the location of the first hole on the front of the Center Spar and measure. In this example it is 85mm from the outside edge, 80mm from the front. The other side should be the same. The line at the bottom of the picture will be the front rivet line.
Back to the first picture. The Red line is the front Center Spar rivet line, the black line above it is the front edge of the Center Spar. The black dots at the bottom represent the backdrilled existing rear Center Spar holes. The Center Spar and front rivet lines are set at the 40mm pitch from the end hole to the center line, the pitch is actually a little bit less than 40mm.
So, the plan is to backdrill the existing rear Center Spar line and then pre-drill the other 2 lines with a 3/32" or #40 drill. Cleco the Doubler to the rear Center Spar and then backdrill through the Completed front Center Spar with a 3/32"/#40 only. This will be easier than using the Right Angle drill attachment.