I have a black easel (Kostiner) and use a white sheet of silky design paper with a grid drawn on it to compose/frame the image and check the black borders. Then I stuck an extra piece of that paper under the grain-focuser (Omega). So, the thickness of that sheet of paper plus the piece under the focuser equals the thickness of FB paper, more ore less.
I think that, particularly while printing larger format negs., the DOF of a reasonably stopped down (LF-) printing lens is not that 'deep', not to mention the circle of diffusion...

About stopping down the aperture for printing, it is the same s*** as while working with a view-camera, after composing/focussing, particularly when age is coming on slowly and one gets easily distracted a little...

Philippe