You've knitted the body, placed your underarm stitches on hold, and now you're motoring up the back toward the neck. Time for some more arithmetic.
1. Calculate the neck width: In general, for a standard crew neck, the neck width is about one third of the body width (not circumference). Example: My sweater has a width of 18 inches. Therefore, the neck of my cardigan needs to be at least 6 inches wide. Of course, you can alter this -- it's your sweater.
2. Calculate how many stitches wide your neck will be: This involves more than simply multiplying your filler stitch gauge by the number of inches of desired width because aran stitches compress stitch gauge. The best thing to do is to lay out your knitting and look at the lower body, which should already have been blocked. Now, measure your desired width across the centre back to figure out how many stitches you need to attain the desired neck width. Example: My gauge in stocking stitch (my filler stitch) is 4 sts per inch which would normally translate to 6" x 4 sts = 24 sts for the neck. BUT, when measured across the centre back with its aran patterns, it turns out that I need 28 sts to achieve 6 inches.
3. Front and back neck depth: For a standard fitting neck, I like to begin the back neck shaping one inch before my final desired body length, and the front neck about 3 inches before the final length. Example: I want my cardigan to be 18 inches long in total. Therefore, I need to start my back neck shaping when my knitting measures 17 inches from the cast on edge. I recommend knitting the back before the fronts; it makes figuring out where to start the front neck shaping simpler. I make notes right on my chart about where I started shaping.
4. Curving the corners: To achieve nice rounded edges on your neck shaping (front and back), allow four stitches on either side of the neck for some strategic decreasing. Decrease one stitch at each side of the neck every row for four rows before going on to complete the neck depth.
5. How to decrease: Using my own cardigan as an example, when I get to the point where I want to start the back neck shaping, I transfer 28 minus (4 x 2) = 20 sts onto a length of waste yarn at the centre back. I then work each side of the back neck separately but at the same time, all on one circular needle with separate balls of wool. It's much easier to decrease into the aran patterns on either side in a consistent manner when this is done. I'll admit, it's the part that I like doing the least, but it's only for a short stretch of work. I like to decrease the two stitches at the very edge of the neck, but you could also work the decreases one stitch in -- knitter's choice. On the right hand side of the neck, facing you, work k2tog on the RS, and p2tog on the WS. On the left side, work SSK on the RS, and p2togtbl on the left. Once you're done the decreasing, make sure to maintain a selvedge stitch at the neck edge all the way to the top.
6. Join the shoulders: When all the neck shaping is done, join the shoulders, right sides together, with a 3-needle BO. Read more about that choice of method here.
Try your cardigan body on now to be sure it fits. You can give it a soak now or wait until the borders are on if you think everything is on track.
While the cardigan was drying, I made some headway with the Ophelia Overalls (they're actually dark navy yarn-dyed cotton/linen, not grey). Here are the hardware options I'm considering: