I have posted some questions regarding this topic here, but I also posted some observations there.
Basically, the Screen Size value the skeletal mesh editor shows is in the range 0-400 or so and is in no way related to the 0-1 value you have to enter into the LOD data for a mesh with LODs. On top of that, the switch value is depended on the vertex count that an object has, so two characters with the same size will not switch at the same distance given the same screen size value if they have vastly different vertex counts. You have to fiddle with dummy objects so that you can spot a LOD switch in the game scene, because what you see in the mesh editor can not be converted to a game scene viewport and I am afraid that even the values I figure out by fiddling around will be invalid when the user plays on a different screen format.
I really think this system is extremely unpleasant to work with and would suggest that a distance based system is at least offered as an alternative. I realize that the eingine had distance based LOD and moved to the screen size system a few updates ago, but I don’t think this is an improvement.