Help needed

In the new Rowan magazine, there's this cute little cardigan. I would really like to do it, but I would like it to be a little shorter. I would like to start mine approximately where the red line is. The problem is, I don't know how to do this because of the curved front pieces. How many stitches do I cast on? How do I know when to start shaping for the armholes? Can anybody help me?



Also, the pattern in my size calls for 13 balls of Cotton Rope. I have 10 (the white ones I showed a while ago). If I manage to make it shorter, do you think 10 is enough or should I get more?