Re: Recessed Light Spacing
think about the bulb you're going to put in it. Find the photometrics or the beam-spread, or something about the light it puts out. Measure the distance (approximately) from the face of the lamp to the floor (generally the same as the ceiling height, but some recessed lamps are deep in the fixture.)
If you want even light on the floor, you'll want some overlap.
If you want even light at waist-height, you'll want a lot more overlap.
Another theory, depending on the look you're trying for, is a whole boat-load of fixtures, all with very low-power lamps (relatively speaking.) That way the light is very smooth and even, and if you loose one or two, you don't have any really dark-spots. (But in a typical room, lights on a 24" grid-center might seem like gross overkill.)
Leo