Figuring out good lightmap settings

I was trying to understand how the generation of lightmaps work and got a good understanding of that in terms of the resulting size on an atlas.

My question is how to figure out a good average atlas size for a game? How to find a good ratio between "Global GI", "Baked GI" and the per Object "Scale in Lightmap"?

I know that "Baked resolution" would give you the base of how many pixels go into each objects lightmap. For the default plane a res of 1 would generate a 10*10 portion in the lightmap. Well that's too low. How about 10? Maybe, you would need to check that out. Let's say 20 is a good amount so shadows cast from different objects would look nice. That would mean that the lightmap would need to be at least 256*256 for the plane to fit.

Is this a good approach? Starting with the largest object and going down from there?