When I focus my adjustable loupe, I make an "X" with a lead pencil on the "lens side" of the ground glass and then focus on that. For me, that was the fastest way to focus the loupe on the grain of the ground glass. I leave the X on the ground glass for checking the loupe's focus, and don't leave the gaffers tape on it more than about a year to avoid tape residue on the loupe.

As for varying the focus of your well-crafted loupe, perhaps the answer is to use inner and outer tubes that are taped together after achieving the proper focus on the ground glass.

Keith