I came across a bit of a problem with this yesterday.
Basically, I cast on for the 47", intending to decrease to the 42.5" through the body of the sweater because I am bottom heavy. I duly worked out that instead of decreasing 2 stitches every 10 rows 4 times, I would decrease 2 stitches every 5 rows 9 times and then not increase as much (I think...still need to work out how many stitches I currently have. This was going to sort out the back perfectly.
However, I did not consider the front. In the original pieced pattern, there is no waist shaping on the front portion and it did not occur to me that I would need to decrease 16 stitches somewhere on the front.
Claire at knitting group encouraged me to drop back and add a decrease, as I contemplated ripping yesterday. I did that; it looks a little loose but I think it will block out and Claire is even more convinced so I am borrowing her confidence. So, in the end, I decreased 4 stitches every 6 rows 4 times - one at either "side" and one at either edge of the cable panel. This will achieve the desired effect and hopefully will not look too bizarre, or it will necessitate ripping back and redoing. I'm surprised to find that, for possibly the first time ever, I'd be willing to rip. It's so lovely that it deserves to be as close to perfect as I can get it.
I should have started increases at 9" and I'm close to 10" so I might end up with it a little longer in the body, which is no bad thing. Another 5ish inches and I will try it on before dividing for the armholes.