Tools
Recessed lighting layout calculator
Room size and ceiling height in, spacing and count out. The wall offset is half the on-center spacing, which is the part most layouts get wrong.
The rule most electricians start from is simple: divide the ceiling height by two, and that is your spacing in feet. An eight-foot ceiling wants cans about four feet apart, a ten-foot ceiling about five. Hold the first row roughly half that distance off the wall so the light washes the wall evenly instead of scalloping the baseboard.
That gets you a grid that will not look wrong. It does not get you a lighting design. Beam angle changes everything, and a 25-degree spot at ten feet covers a fraction of what a 60-degree flood covers. Dark walls, dark floors and tall furniture all eat light that the arithmetic assumes you keep. And a room lit only by a downlight grid tends to look flat and unflattering, which is why good residential schemes layer in sconces, pendants or lamps rather than simply adding more cans.
The footcandle targets behind the lumen figure are typical published values for each room type. The Illuminating Engineering Society publishes the authoritative recommendations, and commercial projects should be designed to those rather than to a rule of thumb.
Before you rely on it
This is arithmetic, not a photometric calculation. It will get you to a sensible starting point and tell you when something is obviously wrong, but a real layout gets run in AGi32 against the actual IES files. Send me the plan and I will run it properly.