As said, it´s not intended to be a precision tool but it´s an improvement from concentric circles reference.
I'm sure it works. But if you make your own tools, try make a sight tube with its length / inner diameter equal to the f/ ratio of your instrument. Put a piece of paper on the mirror box (if one, or otherwise a strip of paper or cardboard piece below the secondary), then move the tube in/out till you see it just frame the secondary with the tiniest line of black between the mirror and its reflection in the inside of the tube. Then adjust the rotation and placement as needed.
I think it is an improvement over even your method - if for nothing else, it uses the true focuser axis. This done, remove the paper, push the sight tube in enough to frame only the reflected primary, and adjust the tilt of the secondary to center it (the reflected primary). This could be done with even greater precision than with the ubiquitous crosshairs, I am sure.
But as with any method, start by tilting the secondary roughly right. The only difficult bit may be finding a piece of tube that fits snugly in your focuser - but once you do, you can easily make a sight tube for each of your Newtonians. To center the peephole, it might be easiest to drill a clean hole (~1.5 mm) in some suitable material, then measure where the outer edge of the tube should be glued.