Layed out rivet lines, marked rivet locations, center punched and drilled to 3/32".
The 94mm end rib trim-back dimension on the lower skin is specifed on drawing from the point where the top skin angle intesects the front web plane. This worked out to be 105mm from edge of the blank. The 87 degree rib angles must be taken into consideration when locating the three holes for the 6W12-1 balance arm. The holes will be in slightly different locations on each attaching rib so it will not be possible to back-to-back match drill them. Image #2 is a simulation of the arm between the ribs at an exaggerated angle. While reviewing the blog of ZAC's 601XL demonstrator upgrade on zenith.aero I noted that the arm appeared to exit the aileron at a right angle to the front web. I'm awaiting a response from Caleb to clarify.