I've had luck with a laser pointer as a tool for judging the proper opening size in the front of the camera. Keep in mind that you want to cover the entire film format, including the corners, which are at the most extreme angle. What I do is an initial cutting (in your case countersinking), then afix a rather large size pinhole to the inside of the opening, with a sheet of white paper the same size as your film format taped to the film plane, and use your laser pointer to see if you can hit the spot of laser light on all 4 corners of the film plane. You also want to make the opening slightly larger than needed, so you don't get grazing angles of light giving you funny reflections, or lint or other kinds of fibers sticking up into the light path and giving the image a fuzzy edge.
~Joe