Neckline Problems

I'm almost done with Mariah, which is good. I thought I wouldn't get done for a while, and it would be spring time before I finished. I really like this pattern, but I noticed something about the cables. My guage is right for the stockinette part, but it's way off for the cables. I had to cut part of the cables off, or the arms would have been too long, and the cabling would have gone on forever. I'm having trouble with the neckline though. I can't figure out how to make it more tapered, so it's more like an ellipse, instead of a circle. I guess I need to look at patterns for other sweaters, to figure out what to do.