Hmm.. I'd imagine that if the citronella oil didn't come with instructions, then you're just supposed to use it straight 100%.....
I bet you could mix it 50/50 or something to extend it a little..
You can wet the wick a little if you want, but it will do that for you really... It's a wick.. lol
It should be sticking out as little as possible while still being able to maintain a flame that you like.