There is no such thing as good practice trying to modify engine files, my suggestion is to create a fork from github and then modify the files as much as you please with your custom version.
Related to your topic, the place you are looking for to modify the light attenuation is inside the DeferredLightingCommon.ush, look at the function GetDynamicLightingSplit. I see you are trying to do some sort of toon effect, check this link, it might help, a bunch of dudes are supporting a toon shading model for UE.