I would spend some time with your mill and make a fixture for holding the couplers. I would make the fixture so that the coupler would be held in place with a screw through the hole in the shank. The fixture should precisely locate the coupler each time one is put in place.
The hole in the coupler then would be located by the Z-Y movement of the mill table and no fiddling would be necessary from one coupler to the next. Use a solid carbide drill because they are stiff and will make a hole where they are pointed, not wander about like a typical twist drill. A really light touch on the Z movement will be essential to avoid going too deep into the coupler.
I cannot answer your question about exactly where the hole should be. It think it would be worth buying a package of couplers with the top operating feature to learn just where and how deep the hole is.
Estes Valley Division