I just got the Clark North American Hammock.  Two years ago was my first Hammock trip.  I loved it, but I was a bit cold.  So, I've read where you can modify hammocks to create more air, or on the Clark, fill the pockets with stuff to make it warmer and that's all fine.  But my question is, what sort of bag rating should I bring in late May or Mid September?  I have a 30 degree bag, and a zero degree bag that both fit into a nice  gore tex bivvy.  I am leaning toward thinking the 30 degree bag should do fine for those dates, but I don't know the answer.  What do you think?