NavList:

Frank said: "If you sweep a great circle from the Sun to the Moon, you will find that it hits the Moon dead-center on the illuminated limb."

That is more easily said than done. But when I am doing a lunar I usually take the telescope out of the sextant to get a good field of view. Then, sighting the Moon through the horizon mirror, I rack the index arm until the Sun appears. The Sun will only appear, however, if I set the sextant perpendicular to the Moon's Horns. I am, effectively, putting the sextant on the plane of the great circle between the Moon and the Sun. 

At that point, of course, I put the telescope back in and make the small adjustment necessary so that the to bodies touch.