The bigger the scale you set, the higher quality assets you will need for the UI to look good.
The scale of the UI depends on the game you have. If your UI assets require pixel perfect scaling, (1, 2, 4, 8, 16, etc), then you will need to probably hard code scale for every device size/density there are.
For example, if your game is targeted for 480x320, the xlarge / xhighdpi device would mean that the scale will probably be 4x (480x4 = 1920 which is full HD device).
If you UI doesn't require pixel perfect scaling, (I think this is your case), then you can just calculate the scale like this and see how it looks. You might want to "clamp" the value if it looks crappy on some devices so that scale is only certain values (1.0, 1.5, 2.0 2.5, 3.0, 3.5)
scale = screen.width / 800;
This should get you started I guess.