When adding cables to a sweater

What is the ‘rule of thumb’ for how many stitches to add to make up for the cable cross-overs? I want to add a 9 stitch braid to a sweater and was wondering what the ratio would be? I heard someone say it is about 50%, ie; on a 6 stitch cable you increase 3 stitches to the over-all width. Is that about right? Any help at the shoulder would (decrease or nevermind) also be wonderful. Because the yarn knits up at 4.5 stitches to the inch that adds 2" to the width and would make the sleeves sag off the shoulder. Would the ‘crossovers’ suck that up or would I look just plain ‘dorky’?

Your increases sound about right. You can decrease in the cable section on the last row, or even on the bind off to bring them back in. It also helps to cross a cable very shortly before bind off.