Nice guide, I'm sure it will be helpful for those still looking. At first I was like "that's excessive use of color" but then I realized... you kept the guide in rainbow color style.
Just two points from my personal experience to add:
1) I did not farm on Normal, but T6 or so. The reason being that if I accidentally engage a RBG (happens really often if you're quick, like with a dash monk who gets through some maps in 2-3 seconds with perma dash) I can poke it every few seconds and prevent it from despawning while inviting other people (and not insta-kill it as on Normal). Of course that only makes sense if you have separate games open and not all 4 of you are looking for RBGs in one game.
2) You might want to clarify very clearly that it's sufficient to be in the party, you don't need to be in the *area* when the goblin is killed. So if you're farming in different games, invite people, and have already engaged the goblin, as soon as the people have entered your game they should be able to get into the portal, even if they're still standing in town. There are lots of people who think you need to be there when the goblin is killed.
As for the goblin spawn chances on certain maps, I'm not so sure about that. I remember seeing lots of different data from different people, but the reason that some areas always pull ahead (like Cave of the Blood Clan or whatever it's called and Leoric's Passage) might be a self-fulfilling prophecy: rumors are it has higher chance to spawn goblins, so more people check it more frequently and more thoroughly, and find more goblins there... rinse repeat. Therefore I'd just pick areas that you like to traverse and most importantly you're very very fast going through (if you often run into dead ends in caves just switch it up, because as you said, efficiency is key). For example, I liked all the tower maps because they're static and with dash/teleport you can make some shortcuts and check 4 individual maps super quickly.