A) By "gut feel" from the corner and then sweeping all the way to the far corner.
B) By gut feel from the "side" (This is Beranek's terminology meaning a few inches from your near corner, then cutting up to it.)
C) By gut feel from the front. (dead center of the undercut, aligning "tilt" and "roll" simultaneously, then cutting up to both corners)
D) By eyeball from the corner. (This method--if used at all--is most commonly done in the tree on spurs and flipline. It involves extensive "cleaning out" of the horizontal undercut with throttle pulses so as to be able to see all the way through the back of the cut. The bar is then "tilted" by feel to provide the desired opening, but the "roll" is established by "sighting down" the [narrow] plane of the bar from the near to the far corner prior to starting the cut.)
I'm thinkin' I lost a few people with that last one. Anyway, what do you guys do? Do you align your back cuts similarly?