If I were doing it, I’d just set up a single cut. Draw a circle the diameter you want the counter bore to be. Put a second circle centered in this circle that’s 8mm in diameter. Put your zero point centered in this circle. Select helical drill for the outer circle, set it however deep you want your counter bore to be. Make a second helical drill for the center circle and make it pop out the other side, give it half a millimeter or so. Now you just mark your extrusion with where you want your holes and center your bit on those holes, rezero and hit go. You could even do what Jeff mentioned, but just put a single piece of straight plywood on the outside of your L. That way you have a reference so all your holes are aligned. If it’s off along the length a little it shouldn’t be too bad, but side to side is where you want it straight.