Interestingly, one of the maps i made was popular primarily for it's background (As well as the action), which i had to do semi-blindly in Fred2. After extensive testing and fixing I found that it worked great. I don't mind having to do it all blindly though.
Usually i don't really need more than 20-25 nebulae's, tops. I'm amazed that the random way of adding numbers can get such amazing results, however.