I'm not experienced enough to help you myself, but whizzball wrote a fairly detailed guide here:Ay, im on the ultimate pack, and attempting to wright a new age using mystcraft. I found the some symbols, but no matter what i do... it seems that they have almost no effect on my world. i added the mushroom biome symbole, and the dense ore symbol, but i got a flat forest with rainbow trees. its like this with all my worlds. how detailed do you have to be in order to make a custom world?
p.s. i already watched direwolf20 and still, i can make customize my worlds.
http://forum.feed-the-beast.com/threads/mystcraft-book-writing-guide.14349/
(It's split over several posts.)
Maybe that will shed some light on the situation...