Mike - that is an excellent technique which I sometimes use as a focus target. The point is to scratch it rather than drawing the line with a pen, so that the crisp edges of the scratch mark comes into acute focus. I use a machinists scribe on an evenly exposed piece of sheet film. But it needs some backup. You also need a magnifier which will focus into the corners of the field and not just near the
center, or at least some good magnifying glasses. Then you need a true machined straightedge to make certain your baseboard isn't warped, to establish how your easel really fits. So far, quite simple.
But for serious work, one wants all three planes aligned all at once. I won't go into detail here, but
anyone who knows how to sight in a gun barrel, using an collimator or laser, and partial-suface mirrors,
will get the clue. It's like extending the line over several hundred feet, bouncing back and forth on a
semi-silvered mirror, that brings everything into alignment within a tiny fraction of a degee. At one time
Salthill marketed a simplified version of this kind of thing for enlargers, and it worked wonderfully. That
was pre-laser. For casual use I also employ a basic 12-inch Stabila level with a machined edge. These
are German-made, not Chinese; and I have some Starrett machinist's levels as backup, which are too
finicky for the uneven surfaces found in most enlargers.