I clamped the predrilled skin on the spar caps so I could put a mark with a sharpie through the first holer on the spar cap. Then used a square to draw a line on the spar cap to indicate where the center of the rib should be. THen clampd the angle peice to the ribs and checked the location so that I could a) get the skin rivit line as close to center as possible while b) getting proper edge distance for the bolt holes on the angles. IIRC I had riveted the angles to the ribs before I did this.
By shifting things one way or the oehr I was able to get enough edge distance on both the angles and the rib flanges. But frankly the angle to spar holes don't line up in the center anywhere near exactly.
In the old old days before predrilled skins you just put the skin holes where they needed to be.