To perfectly set the span wise position of the corner rib I clamped an 0.032 thick 12" steel rule between the rib and a scrap piece of sheet (Photo 1). The longitudinal position is set by lining up the aft edge of the forward outboard flange with the apex of the angle in the bottom skin. I couldn't get a good photo of the bottom, but the shape of the curve in the forward section of the corner rib is a good fit to the curve of that section of the bottom skin. The fit of the aft outboard flange misses the edge of the bottom skin by about 1/8 inch, but this shouldn't matter because the side skin does not rivet to this flange, but curves under and rivets to the web. I used the setup in Photo 1 to match drill halfway back along the inboard row of rivet holes by using a small wood block to hold the rib web tight to the bottom skin. I think I'll wait until I've curled the cone in the side skin to match drill the aft half of the corner rib.
The forward outboard flange of the corner rib curves inboard slightly as it moves aft, so bending the flange puts a slight bow in that section of the rib web which needs to be removed by fluting. With the side skin temporarily attached, I marked the longitudinal positions of the side skin rivet holes so I could flute between them. The bottom edge of that section of the bottom skin is perfectly straight, so I fluted just enough to flatten the web. The clamps shown in Photo 2 hold the bottom skin tight to the rib web for match drilling the outboard row of holes.
Setting the proper span wise and longitudinal positoins of the corner rib.