So what i found out with testing and following this tutorial on this subject "Baked Lighting" and "Lightmaps" (Maya & Unreal Engine) - YouTube,
UnrealEngine4 + Blender - Lightmap Tutorial - YouTube this is the same thing but with blender. In blender you do not need to divide 1 by lightmap resolution. And if you’ll leave 4px of space between islands, it will give better results if quality (engine scalability) is set to low.
There are still issues with those lightmaps when trying to build modular walls like in L shape. But i guess it has to do with need to tweak lighting quality.
It is shame though that you need to go to supposedly outdated tutorial (it’s for UDK not UE4) to get the basics.
Thank you again for pointing me in the right direction.