You don't need to make any edits just make a macro and run this.I prefer making about 3 macros with this and running them.
I'm pretty sure that everything is explained in this guide:viewtopic.php?f=22&t=2357It's not that hard to search a bit.
As fare i know it is hardcoded in client and change by gfx settings.As they doubled the distance in cata. Also for detail doodads.
I don't know if it can work but you can edit a random gluexml lua file, like accountlogin.luaand in the OnLoad part you can put :SetCVar("groundEffectDist", "600");