what file to put float property and properties class blender 2.8 Thanks in advance for any help :)

Premise:
I have three .py files for my blender addon:
1. init.py 
2. panel.py 
3. quadfaces_op.py

I'm trying to make a float slider that controls the face and shape thresholds.
When the code is in one .py file it works well enough. 
Now that I've split the files I cant figure out the following:


Do I need to register the classes in the init.py and/or in the file from which they originate.
When I create a MyVariables(PropertyGroup) my intuition is that it ought to go into panel but it doesn't seem to work...


I've looked at various bits of answers but I can't seem to shoehorn them into what I'm trying to achieve here. 

I if there's some basic premise (what file is inheriting from what) that I can be pointed to for further reading please let me know. The docs have been pretty cryptic so far.
A clean approach for registration is to provide a register and unregister function in every file that contains classes that need to be registered. Provide a top level register and unregister function in the __init__.py which calls the respective functions of the imported modules.

You can put the property groups in a separate file, they don't have to be in the same file as the panels.

A good reference for this approach is the Cycles add-on that can be found in your Blender install [version]/scripts/addons/cycles or intern/cycles/blender/addon in the repository.