I haven't really designed a sweater, but I have given it some consideration and beginnings of attempts. I have the book called Sweater Design in Plain English by Maggie Righetti, which is a good one. So I have read some on this.
What I know is that you will need to do swatches of the different cables and patterns you do to find out how they effect gauge and put that into your calculations for the overall pattern. Cables pull things in a lot. You might try to find Maggie's book or another at the library.