Downlight density (spacing)
I usually approach this (by throwing some light on the subject :)  ) by deciding the activity and necessary light level in the areas concerned.

See:

https://www.archdaily.com/897277/how-to-determine-how-many-led-lumens-youll-need-to-properly-light-your-space

(https://www.noao.edu/education/QLTkit/ACTIVITY_Documents/Safety/LightLevels_outdoor+indoor.pdf  - this is more for a professional situation roads, offices etc)

Then once you have that, you can take the output of the light(s) you intend to use and divide light level by lamp output. This gives you the number of lights.

If they don't fit nicely into the space available then find a light with a higher (or lower) output to get a neat number &amp; spacing.

Of course, you can get away with some margin of error on the numbers but do remember that darker walls tend to need more light.

Also consider that if some need to do fine work (needlework etc) in their comfy chairs then the light level needs to be higher - but you can get that with extra lamps.